A hadith heard directly from the mouth of the Prophet (pbuh&hf) or one of the Imams (a.s) is accepted as is the Holy Quran. As for hadiths received through intermediaries, the majority of Shi`ites act upon them if their chain of transmission is established at every step or if there exists definite proof concerning their truth, and, if they are concerned with principles of doctrine which require knowledge and certainty, according to the text of the Quran.
Other than these two kinds of hadith, no other hadith has any validity concerning principles of doctrines, the invalid hadith being called “tradition with a single transmitter” (khabar wahid). However, in establishing the injunctions of the Shari`ah, because of reasons that have been given, Shi`ites act also on a tradition which is generally accepted as reliable.
Therefore it can be said that for Shi`ism a certain and definitely established hadith is absolutely binding and must be followed, while a hadith which is not absolutely established but which is generally considered as reliable is utilized only in the elaboration of the injunctions of the Shari`ah.