Musannaf Ibn Abi Shayba 13:1387

Abu Bakr narrated to us, he said: Ibn Abi Za’idah narrated to us, from al-Hakam ibn ‘Atiyyah, from Muhammad ibn Sirin, who said: It used to please them that a man should leave for his mukatab (a slave under a contract of manumission) a portion of his kitaba (the payment due for his freedom).