To what extent is the statement attributed to Al-Tabari and Ibn Taymiyyah true, that Hadith are probabilistic in their authenticity from the Prophet, peace and blessings be upon him, and that Ahad (single-chain) narrations do not convey certainty?

Firstly, rejecting Ahadith Ahad (solitary narrations) would invalidate most of the Sunnah and contradict the Quranic command to accept what the Messenger brought. Acceptance is not limited to the scarce Mutawatir (mass-transmitted narrations).

Secondly, definitively established Ahadith include Mutawatir narrations, those narrated by Al-Bukhari and Muslim and accepted by the Ummah, those upon which the Ummah has reached a consensus of acceptance, and widely narrated reports (Khabar Mustafeed) transmitted through many chains. Therefore, not all Ahad narrations are conjectural.

Thirdly, the division of Ahadith into Mutawatir and Ahad does not imply casting doubt on Ahad narrations, and accepting Mutawatir narrations does not contradict relying on Ahad narrations.

Fourthly, the people of Sunnah are unanimous in accepting Ahad narrations in matters of belief, even if they convey only conjecture, contrary to the people of innovation. They hold that it necessitates action without necessarily imparting certainty, and the Ummah is obligated to adhere to it.

Fifthly, Imam Al-Tabari is of the view that a just Ahad report necessitates belief and acting upon it, even if it does not impart certainty, and the Ummah is obligated to adhere to it, whether it pertains to rulings or beliefs.

Sixthly, Ibn Taymiyyah believes that an Ahad hadith may impart certainty if it is accompanied by corroborating evidence. He argues for the absolute validity of Ahad narrations in matters of rulings and beliefs, and that the Ummah's acceptance of Ahad narrations places them at the level of Ijma' (consensus) and necessitates certainty.

In summary: The general body of Muslims accepts Ahad hadith in matters of rulings, and the majority of the people of Sunnah accept them in matters of belief. The issue of whether they are conjectural or definitive does not negate the obligation to believe them and act upon them.