Musannaf Ibn Abi Shayba 13:746

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu’bah, who said: I asked al-Hakam about a man who said to his slave, "Half of you is free." He said: "If it is as you say, that the guarantee is a right, then he is free." And it was the opinion of al-Hakam that he should free him. He said: And I asked Hammad, and he said: "Half of him is freed, and he strives [to earn his freedom] for the remaining half."