Shia Islam is the second-largest branch of Islam. It is rooted in the belief that the Islamic prophet Muhammad explicitly designated his cousin and son-in-law Ali ibn Abi Talib as his rightful political successor (caliph) and the divinely guided spiritual leader of the Muslim community (imam). Shia Muslims maintain that Ali's divine right to leadership was unjustly usurped at the meeting of Saqifa, where certain companions of Muhammad apparently acted against the Prophet's mandate to appoint Abu Bakr as caliph. While Sunni Muslims accept the rule of Abu Bakr, Umar, and Uthman, Shia Muslims remain steadfast in honouring what they perceive to be the Prophet's wishes, recognizing Ali alone as Muhammad's true and legitimate successor.