Sulaiman: What are the sources of Islamic Law according the Shias?
Ja’far: Just like the rest of the Muslims, Shias too believe that the sources of Islamic law are mainly Quran and the Sunnat (practice) of the Messenger of Allah (s.a.w.s.). However according to Shias, the Sunnat of the Holy Prophet (s.a.w.s.) is acceptable only if it has been transmitted through Ahle Bayt (a.s.). This is in consonance of the Prophetic tradition that, "O, people I leave among you those two things that if you remain attached to them you will never go astray, the Book of Allah and my Progeny, my Ahle Bayt."
(Balaghi, Ala ar-Rahman Pg. 44, 2nd edition)
Consequently, for Shias ‘sunnat’ consists of the sayings and actions of the Masoomeen (the fourteen Infallibles). This is because the twelve Imams (a.s.) are the inheritors of Prophetic knowledge and their traditions reach unto the Holy Prophet (s.a.w.s.). As stated by Imam Ja’far as-Sadiq (a.s.),
"My hadith is the hadith of my father (that is Imam al-Baqir a.s.) and the hadith of my father is the hadith of my grandfather and the hadith of my grandfather is the hadith of the Holy Prophet (s.a.w.s.) and the hadith of the Holy Prophet (s.a.w.s.) is the saying of Allah, the Almighty."
(Behaarul Anwaar Vol. 2, Pg. 178)
Therefore according to the Shia fuqaha (jurists) there is no difference between the sayings of the Holy Prophet (s.a.w.s.) and that of the holy Imams (a.s.).