Musannaf Ibn Abi Shayba 13:843

Abu Bakr narrated to us, he said: Abda bin Sulayman and Muhammad bin Bishr narrated to us, from Sa’id bin Abi Arubah, from Qatadah. Muhammad bin Bishr said: From Abu Iyad, from Sufyan al-Uqayli, that Umar said: "Do not buy anything from the slaves of the People of the Covenant (Ahl al-Dhimmah), for they are the people of the land tax (Kharaj), and they sell one another, nor from their land."