Musannaf Ibn Abi Shayba 13:1642

Abu Bakr narrated to us, he said: Mu'tamir ibn Sulayman narrated to us, from his father, he said: Al-Hasan and Ibrahim did not consider it permissible for an *umm al-walad* (a slave woman who has borne a child to her master) to be sold, even if she committed adultery. And Ibn Sirin held the view that she may be sold.