Musannaf Ibn Abi Shayba 18:1248

Abu Bakr narrated to us, he said: Muhammad ibn Bakr narrated to us, from Ibn Jurayj, who said: I said to 'Ata regarding His saying: "The free for the free, and the slave for the slave": He said: If a slave kills a slave like him, then there is retaliation (qisas) for him through him. And if the killer was superior (in status), there is nothing except the value of the one killed.