Musannaf Ibn Abi Shayba 28:907

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