Musannaf Ibn Abi Shayba 13:1916

Abu Bakr narrated to us, he said: Shadhan narrated to us, he said: Yahya bin Hamza narrated to us, from al-Ala’ and al-Nu’man, from Makhul, he said: The *mudabbar* (a slave promised freedom upon the master's death) is from the one-third.