After the death of the Holy Prophet, 'Ammar took part in the apostasy campaigns during the time of Abu Bakr. In the battle of Yamama against the false prophet Musailma when the Muslims began to waver under the pressure of the enemy, 'Ammar mounted a hillock and summoned the Muslims to him. That proved to be a turning point in battle, and the battle ended in victory for the Muslims. When Umar became the Caliph, he appointed 'Ammar b Yasir as the governor of Kufa. He could not hold the office for long and Umar soon deposed him. It is related that when Umar met 'Ammar after his deposition he asked him about his reaction to such deposition he said that he was neither happy when he was appointed as the Governor nor was he happy when he was deposed.