Umar

ʿUmar ibn al-Khaṭṭāb (, also spelled Omar, ) was the second Rashidun caliph. He ruled from August 634 until his assassination in 644. He succeeded Abu Bakr () as the second caliph of the Rashidun Caliphate on 23 August 634. Umar was a companion of the Islamic prophet Muhammad. He was also a father-in-law of Muhammad. Umar was an expert Muslim jurist. He was known for his pious and just nature. It earned him the epithet "al-Farooq". This meand "the one who distinguishes (between right and wrong)".
At first, Umar opposed Muhammad. After his conversion to Islam in 616, Umar became the first Muslim to openly pray at the Kaaba. Umar fought in almost all battles under Muhammad. After Muhammad's death in June 632, Umar pledged allegiance to Abu Bakr () as the first caliph. Umar served as the closest adviser to the Bakr until August 634. It was then that the dying Abu Bakr made Umar his successor.
Under Umar, the caliphate expanded. It ruled the Sasanian Empire and more than two-thirds of the Byzantine Empire. His attacks on the Sasanian Empire resulted in the conquest of Persia in less than two years. Jewish tradition says that Umar set aside the Christian ban on Jews and let them into Jerusalem and to worship. Umar was assassinated by the Persian slave Abu Lu'lu'a Firuz in November 644.
Umar is said to be one of the most powerful Muslim caliphs. He is revered in the Sunni Islamic tradition. Some hadiths say he was second greatest of the Sahabah after Abu Bakr. He is viewed negatively in the Twelver Shia tradition.