Musannaf Ibn Abi Shayba 13:1804

Abu Bakr narrated to us, he said: Ibn Fudayl narrated to us, from Mutarrif, from al-Sha'bi, regarding a man who emancipates his slave at the time of his death, and he has no wealth other than him. He said: He is to be appraised at a fair value, then he (the slave) is to work to earn his value.