Musannaf Ibn Abi Shaybah

It is narrated from Hazrat Hasan (may Allah be pleased with him) that Hazrat Ubaidullah bin Umar (may Allah be pleased with him) was asked: If someone from among the people of Iraq becomes Muslim, what is the ruling regarding him? He (may Allah be pleased with him) replied: If someone from among the people of Iraq who is under our protection becomes Muslim, then his land and his wealth will remain his. But if someone who is not under our protection becomes Muslim—regarding the land which we conquered by force (through jihad)—then that land will belong to the Muslims. Hazrat Ubaidullah says that I had read this ruling in a letter of Hazrat Umar bin Abdul Aziz (may Allah have mercy on him).