Musannaf Ibn Abi Shayba 13:1647

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Yazid bin Ibrahim narrated to us, from Ibn Sirin, from Malik bin ‘Amir al-Hamdani, he said: ‘Umar said regarding the *umm al-walad* (a slave woman who has borne a child to her master): If she remains chaste and remains a Muslim, she is emancipated. And if she commits immorality, disbelieves, and commits adultery, she becomes a slave (again).