Just as Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) did with the land of Iraq and elsewhere: either because it was fay', so he left it as an endowment (waqf, for the Muslims), or because it was spoils of war, so he obtained the consent of those who had conquered it, just as the Messenger of Allah (peace and blessings be upon him) obtained the consent of (those who had taken) the captives of the tribe of Hawazin, until they gave up their rights.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / Q13043