Abdullah bin Sa'id (may Allah have mercy on him) narrates from his grandfather that he heard Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) say on the pulpit of the Messenger of Allah (peace and blessings be upon him): "O company of Muslims, Allah has granted you as spoils from the lands of the non-Arabs, of their women and children, what He did not grant to the Messenger of Allah (peace and blessings be upon him) nor to Abu Bakr (may Allah be pleased with him). I know that men will have relations with these women. So whichever man has a child born to him by one of the non-Arab women, do not sell the ummahat al-awlad (mothers of your children), for if you do, a man may soon have intercourse with a woman forbidden to him without realizing it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21774