Musannaf Ibn Abi Shayba 13:1636

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Ja’far ibn Burqan, from Maymun ibn Mihran, who said: It spread in the camp of Umar ibn Abd al-Aziz that he held the view of permitting the sale of *umm al-walad* (slave women who have borne children to their masters). A man entered upon him and discussed that with him. It turned out that Umar relied regarding their emancipation upon the man who had discussed that with him, and it turned out that Umar held the view that this was the opinion of Umar ibn al-Khattab.