Musannaf Ibn Abi Shayba 18:556

Abu Bakr narrated to us, he said: Hushaym narrated to us, from al-Mughirah, from Ibrahim; and from Yunus, from al-Hasan; and Hajjaj, from 'Ata': They said regarding the one who causes the loss of a fetus: "Upon him is the freeing of a slave along with the *ghurra* (compensation)."