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 (leader) after him, most notably at the event of Ghadir Khumm, but was prevented from the caliphate as a result of the incident of Saqifah. This view primarily contrasts with that of Sunni Islam, whose adherents believe that Muhammad did not appoint a successor and consider Abu Bakr, who was appointed caliph by a group of Muslims at Saqifah, to be the first rightful caliph after Muhammad. A person observing Shia Islam is called a Shi’i.
Shia Islam is based on Muhammad’s hadith (Ghadir Khumm). Shia consider Ali to have been divinely appointed as the successor to Muhammad, and as the first Imam. The Shia also extend this Imammah to Muhammad’s family, the Ahl al-Bayt (“the people/family of the House”), and some individuals among his descendants, known as Imams, who they believe possess special spiritual and political authority over the community, infallibility and other divinely ordained traits. Although there are many Shia subsects, modern Shia Islam has been divided into two main groupings: Twelvers and Ismailis, with Twelver Shia being the largest and most influential group among Shia.