Musannaf Ibn Abi Shayba 19:673

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Sufyan, from Qatadah, from al-Hasan—and it is the statement of Qatadah—that they both used to say regarding a free woman under a slave who commits adultery: the Sunnah for her is that she is stoned. And regarding a free man under whom is a slave woman: he is not stoned.