Musannaf Ibn Abi Shayba 13:1906

Abu Bakr narrated to us, he said: 'Abdah bin Sulayman narrated to us, from Yahya bin Sa'id, from Sa'id bin al-Musayyib, he said: "The *mudabbar* (a slave promised freedom upon the master's death) is from the one-third (of the estate)."