Musannaf Ibn Abi Shayba 18:521

Abu Bakr narrated to us, he said: Yahya ibn Adam narrated to us, from Zuhayr, from al-Hasan ibn al-Hurr, from al-Hakam, from Ibrahim and al-Sha'bi, from Abdullah ibn Mas'ud: That a slave is not to be subjected to retaliation (qisas) from another slave in cases of intentional or accidental injury, except in the case of intentional killing.