Musannaf Ibn Abi Shayba 18:1139

Abu Bakr narrated to us, he said: Muhammad ibn Sawa narrated to us, from Sa’id ibn Abi ‘Aruba, from Qatada, from ‘Umar ibn ‘Abd al-‘Aziz, he said: The injury of the *mukātab* (a slave who has entered into a contract of manumission) is the injury of a slave.