Musannaf Ibn Abi Shayba 19:821

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, from Ruzayq, who said: I wrote to ‘Umar ibn ‘Abd al-‘Aziz regarding a man who accused his son of adultery (qadhf). If the father confesses, [what should be done]? ‘Umar wrote back to him: Flog him, unless he (the son) pardons him.