Musannaf Ibn Abi Shayba 18:789

Abu Bakr narrated to us, he said: Zayd ibn al-Hubab narrated to us, from Ibn Lahi'ah, from Khalid ibn Abi 'Imran, who said: I asked Salim and al-Qasim about a man who killed his slave. They both said: His punishment is that he should be killed, but he is not to be killed in retaliation for him.