The second caliph and Muhammad's father-in-law.
Umar began as an enemy to the nascent Muslim movement. According to Muslim tradition, he converted when he overheard the recitation of the Quran in his sister's house. Thereafter, he was a staunch supporter of Muhammad and the Islamic cause. During the lifetime of Muhammad, he participated in military expeditions and was one of Muhammad's most trusted advisors. At least three Quranic revelations are attributed to his initiative. Upon Muhammad's death in 622, it was Umar who compelled Abu Bakr to accept the caliphate.
Abu Bakr appointed Umar as his successor on his deathbed in 634. During Umar's reign, the Persians were defeated in Iraq and eastern Iran, effectively wiping out the Sassanid dynasty, while the Byzantines were defeated in Syria, Palestine and Egypt. He is credited with the creation of fiscal institutions as well as numerous legal rulings (see "The Four Righteous Caliphs"). He was mudered in 644 by a slave with a personal grudge against him.
Umar was feared rather than loved: he had a harsh disposition and lived an ascetic lifestyle. According to Muslim tradition, he wore patched clothes, ate plain food, and carried a whip while walking in order to chastise those who broke the law.