Musannaf Ibn Abi Shayba 26:125

Yahya ibn Sa'id narrated to us, from Ash'ath, from Muhammad, from Ibn Mas'ud, regarding a man who died and left his father as a slave. He said: "He is to be purchased from his (the deceased's) wealth and then emancipated, and then he inherits." He said: "And al-Hasan used to say this."