Musannaf Ibn Abi Shayba 19:676

Abu Bakr narrated to us, he said: Abda bin Sulayman narrated to us, from Sa’id, from Abu Ma’shar, from Ibrahim, regarding the slave who has a free woman under him, and the free man who has a slave woman under him, and one of them commits adultery. He said: There is no stoning for either of them until they are both free Muslims.