Musannaf Ibn Abi Shayba 13:2249

Abu Bakr narrated to us, he said: Affan narrated to us, he said: Hammad ibn Salamah narrated to us, he said: Humayd informed me, from his grandmother, from Ibn Abbas, that he entered into a *kitabah* (manumission contract) with a slave of his, and he stipulated upon him that he should not work the people hard.