Musannaf Ibn Abi Shayba 19:400

Abu Bakr narrated to us, he said: Asbat bin Muhammad narrated to us, from Mutarrif, from Amir, who said: If it is said to the son of the woman who was subjected to li'an (imprecation), "You are not the son of so-and-so who performed li'an against your mother," he said: The one who says that to him shall be flogged.