Musannaf Ibn Abi Shayba 13:659

Abu Bakr narrated to us, he said: Ibn Mahdi narrated to us, from Sufyan, from al-Zubayr ibn ‘Adi, from Hawt: That a man made a slave woman of his (umm walad) pregnant, and she bore him children. Shurayh said: Her children are in the same status as her; he may employ them, but he may not sell them.