Musannaf Ibn Abi Shayba 18:1300

Abu Bakr narrated to us, he said: Muhammad bin Bakr narrated to us, from Ibn Jurayj, who said: I said to 'Ata': "A slave kills a slave intentionally; the one killed is better than the killer." He said: "The masters of the killed have nothing but the killer of their slave; if they wish, they may kill him, and if they wish, they may enslave him."