Musannaf Ibn Abi Shayba 13:1644

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Sufyan narrated to us, from Abu Husayn, from al-Sha’bi, he said: An *umm walad* (a slave woman who has borne a child to her master) is not enslaved by an occurrence (i.e., she does not become a slave again if she was previously freed, or her status is not altered by the event).