Musannaf Ibn Abi Shayba 13:1645

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Abu Hilal, from Qatada, from Umar ibn Abd al-Aziz, he said: The *umm al-walad* (a slave woman who has borne a child to her master) shall not be sold, even if she commits adultery.