Musannaf Ibn Abi Shayba 19:171

Abu Bakr narrated to us, he said: Ibn Mahdi narrated to us, from Sufyan, from Sa’id al-Zubaydi, from Hammad, from Ibrahim, regarding a man who says to a man, "You are not your father's son," or "Your mother is a slave woman, or a Jewess, or a Christian woman." He said: He is not to be flogged.