Musannaf Ibn Abi Shayba 13:1801

Abu Bakr narrated to us, he said: Hafs narrated to us, from Hajjaj, from Qatadah, from al-Hasan, who said: Ali was asked about a man who emancipated a slave of his at the time of his death, and he had no wealth other than him, and he owed a debt. He said: He is emancipated, and he (the slave) shall work to pay off his value.