Musannaf Ibn Abi Shayba 13:2698

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Sufyan narrated to us, from Isma’il bin Umayyah, from Umar bin Abdul Aziz, that he used to order the selling of the slaves of the People of the Covenant (Ahl al-Dhimmah) if they embraced Islam.