Unit-report (al-khabar al-wahid/pl. al-akhbar al-ahad) constitute almost all transmitted reports (hadiths) compendiums in Islam. Still, there are numerous arguments about the scope of their authenticity among Muslim scholars (‘ulama). The present paper focusing on Shi‘ism (Imamism) first addresses the doctrines of the significant theologians and jurists of the school of Baghdad in the early centuries of Shi‘a-Islam, the school of Hilla in the middle centuries, and 14th century Shi‘a scholars on the epistemological status of al-akhbar al- ahad. It then will explore the central importance of ‘certitude’ (yaqin) in setting up the criteria of the authenticity of the hadiths among the early Imami scholars. This topic is naturally involved with the historical dispute of Usuli jurists and Akhbari. After examining the rationale of the authenticity of unit-report in the legal process and jurisprudence, the paper will deal with a substantial question: was the unit-report among the founder of Imami’s doctrines an authentic and reliable source in other sciences like kalam, divine metaphysics, experimental sciences, e.g. medicine (tibb), economic, sociology, politics and astronomy, or in the issues like the creation of the universe and ontological status of Prophet and Imams?