Musannaf Ibn Abi Shayba 19:401

Abu Bakr narrated to us, he said: Waki’ narrated to us, from ‘Abbad ibn Mansur, from ‘Ikrimah, from Ibn ‘Abbas, who said: Whoever accuses a person of being a child of *li’an* (imprecation) or accuses his mother, he shall be flogged.