Musannaf Ibn Abi Shayba 18:534

Abu Bakr narrated to us, he said: Hafs narrated to us, from Ash'ath, from al-Hakam and Hammad, from Ibrahim: That he said regarding the fetus of a slave woman: From its price, it is like the fetus of a free woman from its blood money: one-tenth and half of one-tenth.