Musannaf Ibn Abi Shayba 19:395

Abu Bakr narrated to us, he said: Shareek narrated to us, from Mughirah, from Ibrahim, and from Layth, from Mujahid and Tawus, regarding a man who says to the son of the woman who was subjected to *li'an* (imprecation), "O son of the adulteress," or who slanders his mother: he is to be lashed.