Musannaf Ibn Abi Shayba 19:166

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 slandered his son. His son said: "If my father is lashed, I forgive him." ‘Umar wrote back to him: "Lash him, unless he (the son) pardons him."