Musannaf Ibn Abi Shayba 13:1809

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Isma'il ibn Salim, from al-Sha'bi, who said: If a man bequeaths the manumission of a slave of his, it is from the third. If it is more than the third, he (the slave) strives to earn the remainder.