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In 644, Umar was murdered by a stab wound from a slave, and the fact that this slave was of Persian origin became a point of Sunni-Shia friction centuries later after Persia converted to Shi'ism under the Safavids. On his deathbed, Umar appointed a committee of six men to find his successor, and this committee included Ali and the eventual choice, Uthman ibn Affan. All six were part of the aging cohort of Sahabah and had been prominent political actors throughout the Prophet's reign and the succeeding caliphs.
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Jesse Harasta (The History of the Sunni and Shia Split: Understanding the Divisions within Islam)