Musannaf Ibn Abi Shayba 7:124

Abu Bakr narrated to us, he said: Umar ibn Ayyub narrated to us, from Ja’far ibn Burqan, from Maymun, that a man had a slave (to be freed) upon him, and he wanted to free the child of a *mukātab* (a slave under a contract of manumission) belonging to them. He said: "I will not free anyone other than him."