Shia (; Shīʿah), an abbreviation of Shīʻatu ʻAlī (, "followers of Ali"), is a branch of Islam which holds that the Islamic prophet Muhammad's proper successor as Caliph was his son-in-law and cousin Ali ibn Abi Talib. Shia Islam primarily contrasts with Sunni Islam, whose adherents believe that Muhammad's father-in-law Abu Bakr, not Ali ibn Abi Talib, was his proper successor.