Sunnah ...
Sunnah , which is often translated as “tradition”, is indeed authoritative for Muslims, but not the sunnah of the clergy, of the seminaries, or of the Muslim community; rather, it is the sunnah of the Prophet (s) that is taken by Muslims as second in authority only to the Qur'an.
In Islamic jurisprudence, four sources of legal rulings are commonly mentioned: the Qur'an the Sunnah reason (Shi'i) or analogy (Sunni) consensus The reliance on consensus in Sunni legal theory is in some respects similar to the Catholic reliance on tradition, although Catholics have focused more on doctrinal issues while Muslims have been more concerned with practical rulings.
However, for the Shi'a, consensus is reduced to the Sunnah , since it is valid only when it unveils the view of the Prophet or the Imams. Hence, for all practical purposes, in present circumstances the sources of legal rulings among the Shi'a are limited to the first three mentioned above: the Qur'an, the Sunnah , and reason.
Sunni and Shi'i Muslims are in agreement that what is meant by the Sunnah is the example of the Prophet (s) in word and deed as recorded and passed down in the form of narrations, called hadiths. For the Shi'a, however, narrations of the words and deeds of the twelve Imams are also taken as authoritative. Sometimes this is justified on the grounds that knowledge of the Sunnah of the Prophet (s) was best preserved in his household, his ahl al-bayt.
Available also on line at: http://www.al-islam.org/a-shiite-creed-shaykh-saduq [^4]: This narration is found in various Shi‘ite as well as Sunni collections of hadiths. See the book: Shi‘ism in Sunnism by Sayyid Muhammad Reza Mudarrisi Yazdi (Qom: Ansariyan, 2003), p. 52. Available on line at: http://www.al-islam.org/shiism-in-sunnism-sayyid-muhammad-ridha-mudarris... [^5]: This narration is reported in Tabari, cited by S. H. M.
Jafri in The Origins and Development of Shi‘a Islam (Qom: Ansariyan, 1989), 179-180.