Musannaf Ibn Abi Shayba 7:121

Abu Qatan narrated to us, from Shu'bah, from Hammad, regarding the *umm al-walad* (a slave woman who has borne a child to her master) in the expiation for *zihar* (a form of divorce oath): He said, "It does not suffice him." And al-Hakam said, "Other than her is more beloved to me than her, and I hope [it is sufficient]."