Shia Islam or Shi'ism is one of the two main branches of Islam. It holds that the Islamic prophet Muhammad designated Ali ibn Abi Talib as his successor and the Imam (spiritual and political leader) after him, most notably at the event of Ghadir Khumm, but was prevented from succeeding Muhammad as the leader of all Muslims as a result of the choice made by Muhammad's other companions at Saqifah. This view primarily contrasts with that of Sunni Islam, whose adherents believe that Muhammad did not appoint a successor before his death and consider Abu Bakr, who was appointed caliph by a group of senior Muslims at Saqifah, to be the first rightful caliph after Muhammad. A person observing Shia Islam is called a Shi'ite or Shi'i.