Musannaf Ibn Abi Shayba 19:410

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Ma’mar, from al-Zuhri, regarding the slave: If he has a free woman under him, then if he accuses her of adultery (qadhf), he is flogged and he does not perform li’an (imprecation). And if he is a free man who has a slave woman under him, and he accuses her of adultery, then he is not flogged and he does not perform li’an. And if he is a slave who has a slave woman under him, and he accuses her of adultery, then he is subjected to the prescribed punishment (hadd) and he does not perform li’an.