Musannaf Ibn Abi Shayba 13:1577

Abu Bakr narrated to us, he said: Humayd ibn Abd al-Rahman narrated to us, from Hasan, who said: I asked Ubayd Allah ibn Umar about those who embraced Islam from the people of al-Sawad. He said: Whoever embraces Islam from the people of al-Sawad who has a covenant (dhimma), then his land and his wealth belong to him. And whoever embraces Islam from those who have no covenant, and it (the land) was taken by force, then his land belongs to the Muslims. Ubayd Allah said: I read this in a book of Umar ibn Abd al-Aziz.