Musannaf Ibn Abi Shayba 13:1521

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Yunus, from al-Hasan, who said: If a slave woman becomes pregnant by her master, then she miscarries, he said: If its creation has become distinct, then she is an *umm walad* (mother of a child), and there is no way to sell her.