What is the definition of Ahad (solitary) narrations, and what is the ruling on acting upon them? What is the ruling concerning one who denies the Prophetic Sunnah that is verbal, claiming that what is mentioned in the Qur'an suffices him? Can examples of Ahad narrations be mentioned?

Linguistically, Ahad is the plural of Ahad. It can mean: "He who has always been alone" (one of Allah's Most Beautiful Names), or it can mean "the one."

Technically, it is a report that does not by itself convey certain knowledge, or it conveys conjecture (dhann), or it is a report that does not meet the conditions of tawatur (mass transmission). It includes the mashhur (well-known), the aziz (rare but with two or more chains), and the gharib (singular).

Evidence for the obligation to act upon a solitary report (khabar al-wahid) has been cumulatively established from the Qur'an and Sunnah, and this is the madhhab of the Companions and those who followed them in goodness. Among the evidence for its acceptance from the Qur'an is Allah’s saying: “And why should not a party from every division of them go forth to acquire sound knowledge in religion” (Surah At-Tawbah, 9:122). The word ta'ifa (party) can refer to a single individual. If a solitary report were not a proof, then this going forth would be meaningless. As for the Sunnah, among the proofs is the Prophet’s sending of teachers and callers to distant lands, such as Mu'adh ibn Jabal. If a solitary report were not a proof, his sending them would have been in vain.

Whoever denies the obligation to act upon the authentic Sunnah is a disbelieving apostate, due to their denial of something known to be an essential part of the religion. Scholars have argued this point based on what Al-Bayhaqi narrated from Imran ibn Husayn (may Allah be pleased with him) when he was asked about Ahadith that have no basis in the Qur'an. He mentioned the prayers and the amounts of Zakat, among other things, arguing with Allah’s saying: “And whatever the Messenger has given you—take it; and what he has forbidden you—refrain from it” (Surah Al-Hashr, 59:7). Deniers of the Sunnah are not “Qur'anists”; rather, they are people of deviation and heresy.