Musannaf Ibn Abi Shayba 18:520

Abu Bakr narrated to us, he said: Muhammad ibn Bakr narrated to us, from Ibn Jurayj, from Abd al-Aziz ibn Umar, that in a book of Umar ibn Abd al-Aziz, from Umar ibn al-Khattab: Retaliation (qisas) is to be taken from a slave for a slave in every intentional act that reaches the soul (killing) and what is less than that.