Musannaf Ibn Abi Shayba 18:519

Abu Bakr narrated to us, he said: Abu Usama narrated to us, from Muhammad ibn ‘Amr, from ‘Umar ibn ‘Abd al-‘Aziz, he said: The slave is to be subjected to retaliation from the slave in every intentional act that reaches the value of himself, and whatever is less than that in terms of wounds.