Al-Hadith As-Sahih Al-Ahad
The Authentic Single Report
In the time of the ghuraba (strangers) and fasad (corruption) of the Deen, people are far way from the wahi (revelation). People will follow the majority and leave the Qur'an and Sunnah, few will hold strong to the Deen.
The Messenger Muhammad ﷺ said,
" بَدَأَ الإِسْلاَمُ غَرِيبًا وَسَيَعُودُ كَمَا بَدَأَ غَرِيبًا فَطُوبَى لِلْغُرَبَاءِ "
Islam start as something strange will return back as something strange, paradise is for the strangers.
The Ghuraba (strangers) are those who reject to be from amongst the deviants. Tuba is a tree in paradise thats width is like the heavens and the earth. That tree is for the believers.
The Evidence for Hadith Ahad
Whether the hadith is mutawattir (famous) or ahad (single report), as long as it is attributed to the Messenger Muhammad ﷺ and is linked in narration from the beginning to the end until it is carried and delivered, it will be taken as an evidence in the Deen of Al-Islam.
At-Talazum (Moulding)
We cannot understand this topic unless we understand At-Talazum (Moulding). This is where two things cannot be separated. Like Ramadan and fasting. Like Iman is sayings and actions with conviction in the heart, Qawol wal Amal wal Itiqaad. Like Aqeedah (belief) and Shariah (Islamic law) are inseparable. Its like you cannot be Muslim unless say Laa ilaha ilallah Muhammadan Rasulullah. To separate these issues is to dismantle the whole matter.
Allah سبحانه و تعالى has names, attributes and functions which cannot be separated. The two pillars of Tawheed, you cannot separate them. There is moulding between the term and its meaning, originally we always take the literal apparent meaning, and we do not go to the metaphoric meaning unless there is evidence. You cannot separate the Qur'an and the Sunnah; there is Talazum between both of them. You cannot separate the Messenger Muhammad ﷺ from the Sahabah رضي الله عنه. We say As-Sahabu Nabi wa Nabi was-sahab (the companion of the Prophet, the Prophet and his companion). There is Moulding between ajal (life) and mawt (death); it is all in the hand of Allah سبحانه و تعالى.
Talazum baina hubbu fillah wal bughdufillah (between love for the sake of Allah and hate for the sake of Allah), we cannot say love for the sake of Allah and then hate the believers. Talazum from Khawf (fear) and raja (hope), whoever fears Allah سبحانه و تعالى will have Taqwa (piety). Talazum baina Qawol Wa Qawol (saying and saying) of the Messenger Muhammad ﷺ. We take the saying of the Messenger Muhammad ﷺ regardless of the number of narrators as long as it is authentically reported. If you separate saying from actions then you are separate Tawheed Rububiyyah (related to command to do) and Tawheed Uluhiyyah (related to doing actions in obedience to Allah).
Allah سبحانه و تعالى says,
وَمَا آتَاكُمُ الرَّسُولُ فَخُذُوهُ وَمَا نَهَاكُمْ عَنْهُ فَانتَهُوا
Whatsoever the messenger gives takes, whatsoever he prohibits abstain from it [Surah Al Hashr 59:7]
People make this separation to say certain evidences are taken into Aqeedah (belief) and other evidences are not. This is because they speak from their own ration (hawa). Hawa is the number one enemy of Wahie.
Allah سبحانه و تعالى says,
وَمَا يَنطِقُ عَنِ الْهَوَى - إِنْ هُوَ إِلَّا وَحْيٌ يُوحَى
He (Muhammad) does not speak from his hawa, it is Wahie revealed [Surah An Najm 53:3-4]
The Prophet (nabi) is the one who Allah سبحانه و تعالى sends the Wahi (revelation). The Messenger (rasul) is the one who passes on the message (risalah) to the people.
Many people say they love the Messenger of Allah سبحانه و تعالى, yet they do not follow hadith Ahad. Either you take from the Prophet of this Deen or you dont. Classify the topic for the sake of clarification by all means but don't separate their importance. So for example when we classify Iman we do not separate it, we say the outer action and the inner action are both part of Iman.
Ilm ul hadith (the science of narration) is studied by two types of scholars; Ahlul hadith and Ahlul Kalam.
When they wrote ilm ul hadith it was in order to classify and not to separate. Either say Qala Rasullallah... the Prophet said or lum ya qula... the Prophet never said. We don't say one Prophet for Aqeedah and one for Sharia'h. We say the Prophet for one Deen.
Any Sahih Hadith checked by the Salaf As-Saleh and then by the Muta-Akhireen (latter scholar), we take it for all Masail (ruling) of the Deen. We say there is no separation between Mutawattir and Ahad hadith. We accept the hadith as long as it is Nabawih As-Sahih and if the Muta-Akhireen and the Ulema of the Salaf if in their own time attributed the hadith to the Messenger Muhammad ﷺ. If the Salaf say Qala Rasulullah... we take it into all matter of the Deen, as they never separate the Aqeedah from the Sharia'h. The later Ulema (scholars) always agreed those who came before them in this regard.
If the most knowledgeable of Ulema agree that the hadith is Sahih, then it is used for all aspects of the Deen. So when we classified the Tawheed we said Laa ilaha ilallah There is none worthy of Worship but Allah Muhammadan Rasulallah Muhammad is the Messenger of Allah, this separation is classification.
If the Muta-Akhireen (later generation) disagrees on an issue, then we go to the Muta-kaddibeen (earlier generation). If Muta-kaddibeen agree it is Sahih (authentic), then who are we to have doubt. The doubt comes from the Shaytan. We dont say we have the Qur'an and Sunnah but we have bidah (innovation). Be Ahlul Ittibah (people who follow).
The Salaf are the first three generation praised by the Messenger Muhammad ﷺ. If the Salaf agree the hadith is Sahih then we take it in all matters of the Deen.
Al-Khabar As-Saheeh Al-Ahad
This is the report that has been attributed to the Messenger Muhammad ﷺ by muntasib (connected) Isnad (chain).
The Deen is Talazum between the belief and the action. Action testifies to a persons belief.
The people who say we do not take it in Aqeedah but accept it in Sharia'h i.e. Hukm Shari, are deviant, thats the difference between Firqa-tu Najiyyah (saved sect) and Firqa-tu Haliyyah (deviant sect).
Questions That Arise
Some people say if the Ahad has doubt then why not follow the Mutawattir and the Qur'an?
What about the one who rejects the khabar Ahad in Bukhari and Muslim?
What is the Hukm for the one who says we need decisive evidence?
The Jumhour (majority) of the Salaf as Saleh were all Ahlul hadith. The Fuqahah (scholars knowledgeable in Islamic ruling) of the Salaf and the Four Imams all agree the Sahih Ahad hadith is accepted in all aspects of the Deen (Aqeedah and Sharia'h) the ones who differ are the deviants, which are those who came after the first three generation.
Allah سبحانه و تعالى says,
يَا أَيُّهَا الَّذِينَ آمَنُوا إِن جَاءكُمْ فَاسِقٌ بِنَبَأٍ فَتَبَيَّنُوا
O believers, if a fasiq (evil person) comes to you with news, verify it [Surah Al Hujurat 49:6]
Once it is verified Sahih (authentic) we take it into the deen. The Shia Rafidi are taifatu-Kufr (group upon disbelief) we take nothing from them in our deen.
The conditions of the person narrating a hadith are that he must be; Muslim, mature, trustworthy and sane, also have a good memory and to keep the words of the hadith verbatim.
Further evidence for Ahad (lone) narration
The Messenger Muhammad ﷺ received Wahi and he was Ahad, like all the Ambiyah. Allah سبحانه و تعالى sent an individual messenger.
Evidence 1
Muadh bin Jabl went to Yemen to teach Islam. The Messenger Muhammad ﷺ said The First thing you call them to is Laa ilaha ilallah
Ahlus Sunnah wal Jama'ah quote Sahabah رضي الله عنه, Tabein and Tabi-Tabein.
Evidence 2
Allah سبحانه و تعالى ordered the Messenger Muhammad ﷺ make tabligh (convey). The tabligh was of the deen so we cannot separate belief and actions. The Salaf As-Saleh transmitted the deen and not just the text from it, by practicing. Thats why no Sahabah رضي الله عنه, Tabein or Tabi-Tabein disagreed in the Azabul qabar (punishment of the grave).
In the issue of al-Asma was-Sifaat of Allah سبحانه و تعالى, the four imams follow the hadith and the ayah and did not say we take Qur'an first and Sunnah second rather followed both together and this should be our understanding too.