Shia Islam is the second-largest branch of Islam. It holds that the Islamic prophet Muhammad designated ʿAlī ibn Abī Ṭālib as his successor (
khalīfa) and the Imam after him, most notably at the event of Ghadir Khumm, but was prevented from succeeding Muhammad as the leader of the Muslims as a result of the choice made by some of Muhammad's other companions (
ṣaḥāba) at Saqifah. This view primarily contrasts with that of Sunnī Islam, whose adherents believe that Muhammad did not appoint a successor before his death and consider Abū Bakr, who was appointed caliph by a group of senior Muslims at Saqifah, to be the first rightful (
rāshidūn) caliph after Muhammad. Adherents of Shia Islam are called
Shia Muslims.