Musannaf Ibn Abi Shayba 13:2194

Abu Bakr narrated to us, he said: Abu Bakr ibn Ayyash narrated to us, from Mughirah, from Ibrahim, who said: If a man enters into a *kitabah* (contract of manumission) with his slave, and he possesses another slave or a slave-girl, then that is from his *kitabah*. But if he has children whom he has concealed, then he does not have that.