Depending on the number of the reporters of the hadeeth in each stage of the isnad, i.e. in each generation of reporters, it can be classified into the general categories of Mutawatir (consecutive) or Ahad (single) hadeeth. Sometimes an ahad route may also be varied depending on the type of transmission as it comes in Ma`rifah Uloom of Imam Hakim and others that even the sole transmission of one town from another town falls into ahad category at times. Ahad narrations are sub-divided into many types such as `Azeez, Mashoor, ghareeb and so on. Each again branching into some more types. But the general definition would be that a report that does not reach the level of tawatur is usually called as khabar-ahad. To those who don’t understand this should first study the books of Mustalahal-Hadeeth
Now this topic should be read such that the view points of both sides of the salaf be known. Those who accept the authority of khabarul wahid and those who restrict it in `aqeedah. Usually while discussing this topic the scholars also quote a huge number of narrations describing the importance of a prophetic tradition. I have skipped that (except very few) and focused on translating and compiling narrations that discuss the issue of mutawatir vs ahad in regards to `aqeedah. This is to convey the jist of the information in a short quick way and also keep it brief. I begin with what
Hafidh ibn Hajar rahimahullah said :
[Ref: Fath ul-Bari 13/234]
Ibn Abil-`Izz al-Hanafi (rahimahullah) said in Sharh `Aqeedah at-tahawiyyah on page 307-8-9 :
An ahad hadeeth, when accepted, believed in and acted upon by the ummah produces certain knowledge according to the majority of scholars of all the ages.  It is treated as a type of mutawatir hadeeth. There is no difference among the Elders on this point. Examples of this kind of hadeeth are many. One is reported by `Umar Ibn Al-Khattab. Its opening words are, “Actions will be judged according to their motives.” Another which is reported by Ibn `Umar says, “The Prophet prohibited the sale of client age as well as its donation.” A third, which is reported by Abu Hurayrah, says, “A woman is not to be married along with her paternal or maternal aunt.” A fourth hadeeth says, “All those marriages which are forbidden because of blood relations are also forbidden on the ground of fostering.” There are many more of this nature. These ahadeeth carry the same weight as the news about the change of qiblah, which was given by one man to the people praying at the Quba’ Mosque, and according to which they turned to the new qiblah which is the Ka’bah. The Prophet used to communicate messages through one man, or send letters through a messenger, but never did anyone say that he would not accept the message because it was passed on by only one man.
[Ref: Sharh `Aqeedah at-tahawiyah]
 For a discussion of this point see Ibn Al-Qayylm, Mukhtasar as-Sawd’iq al-
Moreover, further Imam Ibn Abil-Izz al-Hanafi rahimahullah refuting those who object to ahad narrations said :
This proof is incorrect. It is strange that when someone cites a hadeeth from one or the other of the two Saheeh collections, they object to it and say that is only a solitary report (khabar al-wahid), even though scholars agree that such a hadeeth should be accepted and acted upon. But on the other hand they argue from a poet’s couplet, which is doubtful and not even found in his diwan (collection of poems).
[Ref: Sharh `Aqeedah at-tahawiyah, Page 114]
Imam ash-Shaf`ee even labelled a chapter in his book Kitaab ar-risalah titling it as
الحجة في تثبيت خبر الواحد
“The Hujjah of Khabarul-Wahid”
[Ref: Ar-risalah, page 401]
This is the brilliant narration he brought under this baab smashing every objection against khabarul-wahid
(حديث مرفوع) أَخْبَرَنَا أَبُو عَبْدِ اللَّهِ الْحَافِظُ ، قَالَ : حَدَّثَنَا أَبُو الْعَبَّاسِ ، قَالَ : أَخْبَرَنَا الرَّبِيعُ ، قَالَ الشَّافِعِيُّ قَالَ لِي قَائِلٌ : اذْكُرِ الْحُجَّةَ فِي تَثْبِيتِ خَبَرِ الْوَاحِدِ بِنَصِّ خَبَرٍ أَوْ فِيهِ إِجْمَاعٍ ، قُلْتُ : أَخْبَرَنَا ابْنُ عُيَيْنَةَ ، عَنْ عَبْدِ الْمَلِكِ بْنِ عُمَيْرٍ ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ عَبْدِ اللَّهِ بْنِ مَسْعُودٍ ، عَنْ أَبِيهِ ، أَنّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ , قَالَ : ” نَضَّرَ اللَّهُ عَبْدًا سَمِعَ مَقَالَتِي فَحَفِظَهَا وَوَعَاهَا وَأَدَّاهَا فَرُبَّ حَامِلِ فِقْهٍ غَيْرِ فَقِيهٍ ، وَرُبَّ حَامِلِ فِقْهٍ إِلَى مَنْ هُوَ أَفْقَهُ مِنْهُ ، ثَلاثٌ لا يَغُلُّ عَلَيْهِنَّ : قَلْبٌ مُسْلِمٌ ، إِخْلاصُ الْعَمَلِ لِلَّهِ وَالنَّصِيحَةُ لِلْمُسْلِمِينَ ، وَلُزُومُ جَمَاعَتِهِمْ فَإِنَّ دَعْوَتَهُمْ تُحِيطُ مَنْ وَرَائِهِمْ ” . قَالَ الشَّافِعِيُّ : فَلَمَّا نَدَبَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِلَى اسْتِمَاعِ مَقَالَتِهِ وَحِفْظِهَا وَأَدَائِهَا امْرَأً يُؤَدِّيهَا ، والأمر وَاحِدٌ ، دَلَّ عَلَى أَنَّهُ لا يَأْمُرُ أَنْ يُؤَدَّى عَنْهُ إِلا مَا تَقُومُ الْحُجَّةُ بِهِ عَلَى مَنْ أُدِّيَ إِلَيْهِ ، وَبَسَطَ الْكَلامَ فِيهِ ، وَقَدْ رَوَاهُ هُرَيْمُ بْنُ سُفْيَانَ ، عَنْ عَبْدِ الْمَلِكِ ، وَقَالَ فِيهِ : ” نَضَّرَ اللَّهُ امْرَأً سَمِعَ مِنَّا حَدِيثًا فَأَدَّاهُ كَمَا سَمِعَ ” ، وَبِمَعْنَاهُ رُوِيَ ، عَنْ زَيْدِ بْنِ ثَابِتٍ ، وَالنُّعْمَانِ بْنِ بَشِيرٍ ، عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ .
[Ref: This narration was also brought by Imam Bayhaqi in his Ma`rifatas-sunan wal-Athar under the same chapter الْحُجَّةُ فِي تَثْبِيتِ خَبَرِ الْوَاحِدِ]
As per Imam shaf’`ee whoever did not follow the authentic (and established) prophetic tradition is mentally unstable (crazy or mad) and he considered following khabar ul-wahid as mandatory
[Ref: جماع العلم للإمام الشافعي page 8]
Elsewhere Imam Shaf`ee said to his student Imam rabee` ibn Sulayman al-maradi
“I will teach you something which if Allah wills, will enrich you (and that is that) Never leave the (authentic) prophetic tradition unless another (authentic) tradition is proven to oppose it and then during such differences (which arise) then do as i have taught you ”
[Ref: Manaqib ash-shaf`ee 1/472, with an authentic chain]
Imam ash-shaf`ee (rahimahullah) also said :
لقد ضل من ترك حديث رسول الله صلى الله عليه وسلم لقول من بعده
One who rejects the hadeeth of the messenger of allah (sallalahu `alayhi wa sallam) by giving an argument against it (ta’weel, excuse) after having narrated the hadeeth is a deviant
بل قد قال الإمام أبو عبد الله محمد بن إدريس الشافعي: كل ما حكم به رسول الله صلّى الله عليه وسلّم فهو مما فهمه من القرآن
Imam shaf`ee also said : “Every ruling the prophet sallalahu `alayhi wa sallam derived was (or had a basis) from the Qur`an .”
So, those who argue against khabarul-Ahad, as per this fatwa of Imam shaf`ee it could be argued that even that khabarul-wahid stems from Qur`an and accepting it thus becomes obligatory.
Al-Imam al-Sha`fee said
ﻭﻳﺠﺐ ﺃﻥ ﻳﻘﺒﻞ ﺍﻟﺨﺒﺮ ﻓﻲ ﺍﻟﻮﻗﺖ ﺍﻟﺬﻱ ﺛﺒﺖ ﻓﻴﻪ ، ﻭﺇﻥ ﻟﻢ ﻳﻤﺾ ﻋﻤﻞ ﻣﻦ ﺍﻷﺋﻤﺔ ﺑﻤﺜﻞ ﺍﻟﺨﺒﺮ
It is obligatory to accept a report once it is proven, even if the imams did not act upon this report.
[Ref: Ar-risaala Page 463. Note : this ruling has some exceptions ]
After mentioning this narration, Shaykh يوسف بن عبد الله بن يوسف الوابل in his book أشراط الساعة said :
Imam shaf`ee rahimahullah did not distinguish between mutawaatir and ahad and he did not distinguish between the reports pertaining to `Aqeedah and that pertaining to ordinary practises , rather the main priority (to accept a tradition) was regarding the authenticity of reports [and not something being mutawatir and something ahad]
[Ref: أشراط الساعة 43 Download : http://ia700401.us.archive.org/32/items/aswasw/asw.pdf]
This is true because Imam shaf`ee did not distinguish between mutawaatir and ahad the way it is known today.
Imam Shaafi’ee said in al-Umm 7/283, debating someone saying:
“فقلت له حدد لي تواتر الأخبار بأقل مما يثبت الخبر واجعل له مثالا لعلم ما يقول وتقول قال نعم إذا وجدت هؤلاء النفر للأربعة الذين جعلتهم مثالا يروون واحدا فتتفق روايتهم أن رسول الله – صلى الله عليه وسلم – حرم شيئا أو أحل شيئا استدللت على أنهم بتباين بلدانهم وأن كلا منهم قبل العلم عن غير الذي قبله عنه صاحبه وقبله عنه من أداه إلينا ممن لم يقبل عن صاحبه أن روايتهم إذا كانت هذا تتفق عن رسول الله – صلى الله عليه وسلم – فالغلط لا يمكن فيها قال فقلت له لا يكون تواتر الأخبار عندك عن أربعة في بلد ولا إن قبل عنهم أهل بلد حتى يكون المدني يروي عن المدني والمكي يروي عن المكي والبصري عن البصري والكوفي عن الكوفي حتى ينتهي كل واحد منهم بحديثه إلى رجل من أصحاب النبي – صلى الله عليه وسلم – غير الذي روى عنه صاحبه ويجمعوا جميعا على الرواية عن النبي – صلى الله عليه وسلم – للعلة التي وصفت قال نعم لأنهم إذا كانوا في بلد واحد أمكن فيهم التواطؤ على الخبر ولا يمكن فيهم إذا كانوا في بلدان مختلفة فقلت له لبئسما نبثت به على من جعلته إماما في دينك”
Nu`aym bin hammad rahimahullah said :
من ترك حديثا معروفا فلم يعمل به , وأراد له علة أن يطرحه فهو مبتدع
Whoever rejects a ma’roof (well know or established) hadeeth and does not act upon it and makes an intention to remove a fault in this hadeeth so as to excuse him from following it then he is a mubtadee’ (innovator)
[Ref : al-faqeeh wal mutafaqih lil Khateeb – 1/384, saheeh. Once again this along with many other umpteen narrations is a proof that the distinguishing between mutawaatir and ahad , even if right or okay, should not be taken as an excuse to reject itiqad based traditions]
And from this are the narrations that the sahaaba narrated to each other. And when any of them (radhiallahu `anhum) used to narrate to someone from the Messenger of Allah (sallalahu `alayhi wa sallam) then no one would deny that narration saying that this report of yours is khabarul-wahid and until it doesn’t reach (the level of) tawatur this knowledge is of not much benefit ….
And if one of them (the sahaba) were to narrate a hadeeth from the Messenger of Allah (sallalahu `alayhi wa sallam) about the attributes of Allah then he would definitely and most certainly accept and believe in it such as the attributes of
(i) seeing (Allah on qiyamah);
(ii) the kalaam of Allah
(iii) the calling of Allah towards his slaves on the day of judgement with a voice (or call) (iv) that both the ones in distant and near could hear it
(v) Him (subhanahu wa ta`la) descending in the lower heavens every night
(vi) His laughter
(vii) His happiness
(viii) Him holding the heavens from a finger among the fingers of his hand
(ix) Establishing (the belief in) his feet
and thus whoever heard these narrations from one who narrated it from the Messenger of Allah (sallalahu `alayhi wa sallam) or from one of the sahaaba then along with having just and trustworthy narrators (narrating these reports) he affirmed his belief in these ahadeeth and did not have any doubts in it.
Rather that sometimes they did express their doubts over various issues of Ahkaam but no one sought for other evidences when they recieved (an authentically established) hadeeth of sifaat ; rather they were the foremost to accept it upon hearing it, to affirm it….and they established the attributes (of Allah) using those (khabar ul-wahid) narrations (which they heard or read).
And whoever has even a little to do with the prophetic narrations and their whereabouts knows this (speech is true) and had it not been that this (proof) was sufficient (in itself) then we would have given more than one- hundred (100) examples (to prove this, but this example above itself suffices)
As for those who adopted the (strange) principle of believing that definite knowledge is not obtained from (ahad) prophetic traditions have opposed the consensus of the sahaaba on something (so obvious, crucial and basic) that it is known by necessity , and have opposed the consensus of the Taba`een and the consensus of the Imams of Muslims and thus subsequently have fallen in agreement with the stance of what the Mu`tazili, the Jahmi , Rawafidh and Khawarij hold it to be prohibited to take aqaid issues from khabarul wahid .and some usooliyeen and fuqaha happened to follow their stances otherwise no one among the salafus-saaliheen coincided or agreed to their (false) stance (of rejecting khabarul wahid)
Rather the leaders of scholars have opposed them such as Malik, shaf`ee, the companions of Abu Haneefah, Dawood ibn `Ali and his companions such as Abu Muhammad ibn Hazam who said khabarul-wahid gives the benefit of definite and certain knowledge.
[Ref: Mukhtasar as-sawaiq 2/361-362]
6- صح إجماع الأمة كلها على قبول خبر الواحد الثقة عن النبي(ص), وأن جميع أهل الإسلام كانوا على قبول خبر الواحد الثقة عن النبي(ص), يجري على ذلك كل فرقة في علمها, كأهل السنة والخوارج والشيعة والقدرية, حتى حدث متكلموا المعتزلة بعد المائة من التاريخ, فخالفوا الإجماع في ذلك.
Imam ibn Atheer (D. 606. A.H) said :
وذهب قوم إلى أن العقل يدل على وجوب العمل بخبر الواحد، وليس بشيء، فإن الصحيح من المذهب والذي ذهب إليه الجماهير من سلف الأئمة من الصحابة والتابعين والفقهاء والمتكلمين: أنه لا يستحيل التعبد بخبر الواحد عقلاً. ولا يجب
التعبد به عقلاً، وأن التعبد واقع سمعًا، بدليل قبول الصحابة لخبر الواحد، وعملهم به في وقائع شتى لا تنحصر، وإنفاذ رسول الله – صلى الله عليه وسلم – رسله وقضاته وأمراءه وسُعَاته إلى الأطراف، وهم آحاد، وبإجماع الأمة على أن العامي مأمور بإتباع المفتي وتصديقه، مع أنه ربما يخبر عن ظنه، فالذي يخبر عن السماع الذي لا شك فيه أولى بالتصديق.
Some scholars denied the obligation of acting according to the khabarul wahid but their view is wrong and the correct view according to the Pious Predecessors, the Companions, the Taba`een, the Fuqaha, and theologians is that it is permissible to act according to the single chain of transmission in the acts of worship. The proof is: The Sahaaba accepted khabarul wahid and acted according to it in a number of incidents. Moreover, the Prophet (sallalahu `alayhi wa sallam) sent his messengers, judges, and Amirs to remote areas while they were single narrators who carried the rulings and commands. It is unanimously agreed that the public are commanded to follow and believe the Mufti (one who issues a religious ruling) despite the fact that a Mufti may be judging according to his supposition in matters that do not have texts in the Shari`ah, so the (trustworthy) narrator who reports a narration he heard is more reliable to be believed.
According to some , even denying one Ahad Hadith (after knowing its authenticiy) is Kufr :
سَمِعْتُ الشَّيْخَ أَبَا بَكْرٍ أَحْمَدَ بْنَ إِسْحَاقَ الْفَقِيهَ وَهُوَ يُنَاظِرُ رَجُلًا، فَقَالَ: الشَّيْخُ: حَدَّثَنَا فُلَانٌ، فَقَالَ لَهُ الرَّجُلُ: دَعْنَا مِنْ حَدَّثَنَا، إِلَى مَتَى حَدَّثَنَا، فَقَالَ لَهُ الشَّيْخُ: «قُمْ يَا كَافِرُ، وَلَا يَحِلُّ لَكَ أَنْ تَدْخُلَ دَارِي بَعْدَ هَذَا»
Imam Hakim narrated from his Sheikh Abu Bakr Ahmad bin Ishaq Al Faqeeh and he was debating a person , the Sheikh said narrated to us so and so, thereupon the person said “Leave it narrating to us, from upto where it has been narrated” to which Sheikh said to him : Stand Up Oh Kafir (Disbeliever) it is not permissible for you to enter my house after this.
[Ref : Marifat Uloom al Hadith pg 3. Via Muhammad Kashif Khan who said Allama Zubayr Ali Zae narrated to me this when i asked him about performing takfeer of the one who denies even single authentic hadith (on stupid excuses and usool).]
Imam Bukhari titled a chapter in his saheeh :
بَابُ مَا جَاءَ فِي إِجَازَةِ خَبَرِ الْوَاحِدِ الصَّدُوقِ فِي الأَذَانِ وَالصَّلاَةِ وَالصَّوْمِ وَالْفَرَائِضِ وَالأَحْكَامِ
Allamah Abu Waleed Al Baji (D 474 H) al-Maliki al-Andalusi – who was the contemperory of Imam ibn Hazam – said :
( ذهب القاساني وغيره من القدرية إلى انه لا يجوز العمل بخبر الآحاد ….. والذي علية سلف الأمة من الصحابة والتابعين والفقهاء , انه يجب العمل به . والدليل على ذلك إجماع الصحابة على صحة العمل به ….)
Al Qasani and others from the Qadriyah (a deviant sect) went to the opinion it is not permissible to act upon Khabar al Wahid…
Upon which is the Nation of Salaf amongst the Companions, Tabaen and Jurists, that it is Obligatory to act upon it. And the proof for acting upon it is the Consensus of Companions on its validity.
(Ahkaam al Fusul pg 334, Translated by kashif (Muhammad khan))
Ahad hadith is accepted in both aqaid and amal upon which is the Consensus of Sahaba radi Allahu anhum
REPLYING TO SOME OF THEIR OBJECTIONS
1. They use the verse ” And they have thereof no knowledge. They follow not except assumption (az-zann – الظَّنَّ or probability), and indeed, assumption avails not against the truth at all. ” [Qur`an 53:28]
Using these verses to reject khabarul-wahid saying it gives only probably (dhanni) ilm then this is rejected because this verse is talking about the lies, false assumptions and it’s kind whereas a hadeeth which is authentically established does not fall under lies or assumptions or even probabilities
Imam ibn Atheer rahimahullah in his Gahreebul-Hadeeth wal-Athar (3/162-163) and in other books including Lisan ul-`Arab has a different defnition of adh-dhann than what they assume. In these books it is summarized as such.
I have translated the dars of Shaykh rafeeq tahir who refuted this claim used by Munkirul hadeeth as well as those who use this verse as an argument to deny khabarul waahid here : http://fahmalhadeeth.blogspot.com/2015/06/refuting-doubts-of-munkirul-hadeeth-in.html
Related materials : Shaykh Albani’s risalah defending khabar waahid/ahad narrations in english : https://islamfuture.files.wordpress.com/2009/11/hadith-is-proof-itself-in-belief-laws.pdf