Musnad Ahmad 28:550

Ya'qub narrated to us, my father narrated to us, from Muhammad ibn Ishaq, who said: 'Amr ibn Shu'ayb mentioned, from his father, from his grandfather, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, decreed regarding the children of those who have performed li'an (mutual cursing) that he inherits from his mother, and his mother inherits from him. And whoever accuses her of having him [illegitimately] shall be flogged eighty lashes, and whoever calls him a child of adultery shall be flogged eighty lashes."