After the assassination of the third caliph Uthman in 656 by provincial rebels, the caliphate fell into civil war as Mu'awiya ibn Abi Sufyan, a relative of Uthman and the governor of Syria, challenged the legitimacy of the new caliph Ali. The indecisive battle between the two at Siffin ended in an arbitration agreement in July 657. Asserting that human arbitration was invalid as God's command was clear that the rebels (in this case Mu'awiya) had to be fought and overcome, some of Ali's soldiers left the army. They were called Kharijites following this secession. Abu Bilal's brother Urwa ibn Udayya is reported to have been the first person to raise the slogan of "la hukma illa li-llah" (judgment belongs to God alone), which later became the characteristic Kharijite slogan, against the arbitration. Abu Bilal himself was present at the battle and was among the seceders. He later fought against Ali in the Battle of Nahrawan in July 658 where the caliph crushed the Kharijite insurgents. Following Ali's assassination in 661 by a Kharijite, Mu'awiya became the sole ruler, establishing the Umayyad Caliphate.