Musannaf Ibn Abi Shayba 13:1524

Abu Bakr narrated to us, he said: Ma'n ibn 'Isa narrated to us, from Ibn Abi Dhi'b, from al-Zuhri, who said: The *umm al-walad* (slave woman who has borne a child to her master) is emancipated if she miscarries, if it is known that it was a miscarriage.