The succession to Muhammad is the central issue that split the Muslim community into several divisions in the first century of Islamic history, with the most prominent among these sects being the Shia and Sunni branches of Islam. Sunni Islam maintains that Abu Bakr was the legitimate successor to Muhammad on the basis of election. Shia Islam holds that Ali ibn Abi Talib was the designated successor to the Islamic prophet Muhammad. The contrasting opinions regarding the succession are primarily based on different interpretations of the events in early Islamic history as well as of hadiths (sayings of Muhammad). The Sunni believe that Muhammad had not appointed a successor and had instead intended for the Muslim community to choose a leader from among themselves. They accept the rule of Abu Bakr, who was elected at Saqifah, and that of his successors, who are together referred to as the Rashidun Caliphs. On the other hand, the Twelver Shia believe that Ali had previously been nominated by Muhammad as his heir, most notably during the Event of Ghadir Khumm, following the revelation of verse 5:67 of the central religious text of Islam, the Quran. They view the rulers who followed Muhammad as illegitimate. Instead, the rightful successors of Muhammad are believed to be Ali and eleven divinely-appointed Imams of his lineage. In Twelver Shia belief, the last of these Imams, Mahdi, went into occultation in 260 AH (874 CE), compelled by the hostility of his enemies. The advent of Mahdi is awaited by most Muslims, though different sects hold different views about him. Historiography Most of the Islamic history was transmitted orally until after the rise of the Abbasid Caliphate. Historical works of later Muslim writers include the traditional biographies of Muhammad and quotations attributed to him—the sira and hadith literature—which provide further information on Muhammad's life. The earliest surviving written sira (biography of Muhammad) is Sirat Rasul Allah (Life of God's Messenger) by Ibn Ishaq (d. 761 or 767 CE). Although the original work is lost, portions of it survive in the recensions of Ibn Hisham (d. 833) and Al-Tabari (d. 923). Many scholars accept these biographies although their accuracy is uncertain. Studies by Schacht and Goldziher have led the scholars to distinguish between legal and historical traditions. According to Watt, although legal traditions could have been invented, historical material may have been primarily subject to "tendential shaping" rather than being