Musannaf Ibn Abi Shayba 18:546

Abu Bakr narrated to us, he said: Abu Usamah narrated to us, from Abd al-Malik, from Ata, who said: The Messenger of Allah (may Allah bless him and grant him peace) said regarding the fetus: "A ghurra (compensation) of a male slave, or a female slave, or a mule."