Musannaf Ibn Abi Shayba 18:515

Abu Bakr narrated to us, he said: Ma'n ibn 'Isa narrated to us, from Ibn Abi Dhi'b, from al-Zuhri, who said: There is no *qisas* (retaliation) between a free person and a slave, except that if a slave kills a free person, he is killed for him.