Yahya bin Bukayr said: The Shura (consultation) took place, and the people agreed upon Uthman (may Allah be pleased with him) (for the caliphate) when three days remained of Dhu al-Hijjah in the year 23 AH. Then Uthman (may Allah be pleased with him) was martyred on a Friday, when eighteen (18) days of Dhu al-Hijjah had passed, at the close of the year 35 AH. He was eighty-eight (88) years old, and he used to dye his beard yellow. The caliphate of Uthman (may Allah be pleased with him) lasted twelve years.
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 106