The tradition preserved its own worst period rather than tidying it, which is worth noticing. Uthman, the third caliph, was besieged in his house by Muslim rebels from Egypt and Iraq and killed in 656 while reading the Quran. Ali, the fourth, faced immediate civil war: at the Battle of the Camel he fought a force that included Aisha, the Prophet's widow; at Siffin he fought Muawiya's Syrian army to a standstill and accepted arbitration, which caused a portion of his own supporters, the Kharijites, to break away and declare him an unbeliever. One of them assassinated him in the mosque at Kufa in 661. Umar, the second caliph, had already been stabbed while leading prayer. Three of the first four caliphs were murdered, two of them by Muslims over political legitimacy, within thirty years of the Prophet's death. This is the origin of the Sunni–Shia division and of the Kharijite logic that modern militant takfir descends from, and every Muslim historian has had to write about it.