Musannaf Ibn Abi Shayba 13:1519

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from his father, from Ikrimah, who said: Umar ibn al-Khattab said regarding the *umm al-walad* (a slave woman who has borne a child to her master): Her child has emancipated her, even if it was a miscarriage.