Musannaf Ibn Abi Shayba 13:744

Abu Bakr narrated to us, he said: Asbat bin Muhammad narrated to us, from Mutarrif, from al-Sha’bi, regarding a man who emancipated one-third of his slave: He said: He (the slave) shall work to earn the remaining two-thirds for him, and he (the master) is not liable for the remainder of him.