He led a delegation of seventy Kindite warriors to the Islamic prophet Muhammad and converted to Islam. Following Muhammad's death in 632, he and his tribe withheld the customary payment to the nascent Muslim state and apostasized during the Ridda wars. After their subsequent defeat at the Zurqan pasture in Hadhramawt, they and their families took up position at the al-Nujayr fort in Yemen and were besieged by the Muslim forces. Al-Ash'ath secured safe passage for a number of his relatives, but the rest of the besieged fighters were executed. He was spared but taken captive and sent to Caliph Abu Bakr (r. 632–634), who agreed to release him after he repented. He thereafter took up residence in Medina, capital of the caliphate, where he was married to Abu Bakr's sister, Umm Farwa. Despite an initial prohibition on former apostates participating in the Muslim conquests, during the reign of Caliph Umar (r. 634–644), al-Ash'ath joined the Muslim army of Sa'd ibn Abi Waqqas, which was dispatched to conquer Iraq from the Sasanian Empire. During that campaign, he fought in the battles of Qadisiyya, Ctesiphon, Jalula and Nihawand between 636 and 642. During the conquest of these territories, he gained a piece of land in the Arab garrison town of Kufa, at the time of its founding. He built a house there and it formed part of the Kindite neighborhood.