Musannaf Ibn Abi Shayba 13:1643

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Aban ibn Sam’ah, from Bakr ibn Abdullah al-Muzani, he said: Umar ibn Abd al-Aziz wrote regarding an Umm Walad (a slave woman who has borne a child to her master): She is free, even if she commits adultery.