ʾUmar ibn al-Khattāb (d. 644 (AH 23)). Second caliph (khalīfa) and main architect of the Arab Islamic empire. Originally he was an enemy of Islam, but had a sudden conversion four years before the hijra. In Madīna, ʾUmar's energy of will, piety, wisdom, and organizing ability made him second to the Prophet Muḥammad in authority and prestige. The Prophet nicknamed him faruq, ‘distinguisher’ (between truth and falsehood). As second caliph (khalīfa) he organized the Islamic conquests and the administration of the empire. Traditions reveal, however, that he was feared rather than loved, and at the height of his power he was assassinated at Madīna.

It was during ʾUmar's caliphate that Muslim religious and political institutions arose which were to be the model for future generations. Among these were: the dīwān (‘stipend register’), a form of welfare state by which annual stipends were paid to all Muslims from the public treasury; regulations for non-Muslim subjects (dhimmi); military garrisons which later became the great cities of Islam, e.g. Kūfā and Fustat; the office of qāḍi (judge); religious ordinances such as obligatory nightly prayers in the month of Ramaḍān; civil and penal codes; the hijra calendar; and the standardization of the text of the Qurʾān.

Orthodox Sunni sources praise ʾUmar for piety, justice, and make him a model for all the virtues of Islam. In contrast, Shīʿa sources retain an animus against the man who blocked the claims of ʿAlī.

Omar (active c.581–644) ( Umar) Second Caliph, or ruler, of Islam. He was converted to Islam in 618, and became a counsellor of Muhammad. In 632, he chose the first caliph, Abu Bakr, and succeeded him in 634. Under his rule, Islam spread by conquest into Syria, Egypt, and Iran, and the foundations of an administrative empire were laid.