Musannaf Ibn Abi Shayba 9:154

Hushaym narrated to us, from Abu Bishr, from Sa’id ibn Jubayr, regarding the free man who marries a slave woman. He said: "It is not far removed from fornication, except for a little, due to His saying: 'And that you be patient is better for you.'" He said: "Regarding the marriage of a slave woman."