Musannaf Ibn Abi Shayba 13:3403

Abu Bakr narrated to us, he said: Yahya ibn Sa'id al-Qattan narrated to us, from Ash'ath, from al-Hasan, regarding a man who said to his slave, "Serve me for a year and you are free." He said: He serves him for a year and he is free. And if he said, "You are free on the condition that you serve me for a year," then the man died, he serves his children for a year after him, and he is emancipated from his [the deceased's] one-third.