Umar ibn al-Khab (Arabic: , c. 583/584 3 November 644), also known as Umar or Omar, was the second Rashidun caliph. He was one of the most powerful and influential Muslim caliphs in history. He was a senior companion and father-in-law of the Islamic prophet Muhammad. He succeeded Abu Bakr (632634) as the second caliph of the Rashidun Caliphate on 23 August 634. He was an expert Muslim jurist known for his pious and just nature, which earned him the epithet al-Farooq ('the one who distinguishes (between right and wrong)'). Under Umar, the caliphate expanded at an unprecedented rate, ruling the Sasanian Empire and more than two-thirds of the Byzantine Empire. His attacks against the Sasanian Empire resulted in the conquest of Persia in less than two years (642644). According to Jewish tradition, Umar set aside the Christian ban on Jews and allowed them into Jerusalem and to worship. Umar was assassinated by the Persian slave Abu Lu'lu'a Firuz in 644. Umar is revered in the Sunni tradition as a great ruler and paragon of Islamic virtues, and some hadiths identify him as the second greatest of the Sahabah after Abu Bakr.