Musannaf Ibn Abi Shayba 13:1908

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from Ash'ath, from al-Sha'bi, that Ali used to consider the *mudabbar* (a slave promised freedom upon the master's death) as coming from the one-third (of the estate). And that 'Amir (al-Sha'bi) used to consider it as coming from the one-third.