Details about Muslim's early life and career are scant. He was most likely born before the Hijra in 622, the start of the Islamic calendar. He was the son of a certain Uqba of the Banu Murra, a branch of the Arab tribe of Ghatafan. Muslim most likely moved to Syria from Arabia during the Muslim conquest of the region in the 630s. He became a committed partisan of the Umayyad clan of the province's governor, Mu'awiya ibn Abi Sufyan. During the First Muslim Civil War, Muslim distinguished himself at the head of a contingent of Syrian infantry in Mu'awiya's army at the Battle of Siffin in Upper Mesopotamia against Caliph Ali ( 656–661) and his supporters. However, he was unable to wrest control from the latter of the Dumat al-Jandal oasis in northern Arabia during a later battle. When Mu'awiya became caliph in 661, he gave Muslim the lucrative post of land tax collector in Palestine, though he was known not to have taken advantage of this post for self-enrichment. Later, when Mu'awiya was on his deathbed, he made Muslim and the governor of Damascus, al-Dahhak ibn Qays al-Fihri, regents until his son and chosen successor, Yazid I( 680–683), returned to Syria from the battlefield with the Byzantines in Anatolia.