Sunni and Shia hadith collections differ because scholars from the two traditions differ as to the reliability of the narrators and transmitters. Narrators who took the side of Abu Bakr and Umar rather than Ali, in the disputes over leadership that followed the death of Muhammad, are seen as unreliable by the Shia; narrations sourced to Ali and the family of Muhammad, and to their supporters, are preferred. Sunni scholars put trust in narrators such as Aisha, whom Shia reject. Differences in hadith collections have contributed to differences in worship practices and shari'a law and have hardened the dividing line between the two traditions.