Musannaf Ibn Abi Shayba 19:398

Abu Bakr narrated to us, he said: Abu Bakr ibn Ayyash narrated to us, from Musa ibn Ubaydah, from Nafi’, from Ibn Umar, he said: Whoever accuses the son of the woman who performed li’an (imprecation) [of illegitimacy], he shall be flogged.