Abu Bakr (ä´bōō bäk´ər), 573–634, 1st caliph, friend, father-in-law, and successor of Muhammad. He was probably Muhammad's first convert outside the Prophet's family and alone accompanied Muhammad on the Hegira. The marriage of Abu Bakr's daughter Aishah to Muhammad made the ties even stronger. On the Prophet's death in 632, Umar secured Abu Bakr's election over the tribal chiefs and Ali. The two years of his caliphate were critical for Islam. Though he was himself fervent rather than warlike, his party crushed opposition in Arabia and began the remarkable extension of Islam as a world religion. He was succeeded by Umar.