Musannaf Ibn Abi Shayba 13:823

Abu Bakr narrated to us, he said: Hushaym narrated to us, from al-Shaybani, from al-Sha’bi, that he used to say regarding a man who had an obligation of [freeing] a slave, so he bought one and stipulated upon him [the slave] that he would free him: He said: He disliked that and said, "It is not complete."