Musannaf Ibn Abi Shayba 13:1813

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, he said: Hisham was asked about a man who emancipated a portion of a slave belonging to him at the time of his death. So he narrated to us from Hafs ibn Sulayman, from al-Hasan, that he said: It is [taken] from his one-third; it does not exceed that.