Umar ibn al-Khaṭṭāb was the second Rashidun caliph, reigning from 634 until his assassination in 644. He succeeded Abu Bakr (632–634) as the second caliph of the Rashidun Caliphate on 23 August 634. Umar was a senior companion and father-in-law of the Islamic prophet Muhammad. He was also 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)"). An arbitator of the Adi clan, Umar initially opposed Muhammad, his distant Qurayshid kinsman. After his conversion to Islam in 616, he became the first Muslim to openly pray at the Kaaba. Umar participated in almost all battles and expeditions under Muhammad, who bestowed the title Al-Farooq ('the Distinguisher') upon Umar, for his judgements. After Muhammad's demise, Umar pledged allegiance to Abu Bakr as the first caliph, and served as a close advisor to the latter until his death in 634, when Abu Bakr nominated Umar as his successor. 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 (642–644).