Events leading to Ali's assassination trace back to the death of Muhammad, prophet of Islam, where the community of Muslims disputed over his succession as the leader of Ummah. The assembly at the Saqifat Bani Saida gave allegiance to Abu Bakr as the caliph. While Sunni muslims believed that Muhammad had not selected a successor, Shia Muslims believed that Ali was appointed as Muhammad's successor by God referring to the event of Ghadir Khumm. Abu Bakr was succeeded by Umar, who was murdered in 644. After Umar's death, Ali, the cousin and son-in-law of Muhammad, and Uthman were contenders for the post. Uthman was elected as caliph by an election council. Ali became the caliph after the assassination of Uthman in 656.