Musannaf Ibn Abi Shayba 19:489

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Ma’mar, from al-Zuhri, who said: If a man has intercourse with his slave woman while she has a husband, he shall be flogged one hundred [lashes], whether he is married (muhsan) or not married. If she becomes pregnant, the child belongs to the bed (i.e., the husband).