Musnad Ahmad 5:618

Muhammad ibn Ja‘far narrated to us, Sa‘id narrated to us, from ‘Abdullah al-Danaj, from Hudayn, who said: It was testified against al-Walid ibn ‘Uqbah in the presence of ‘Uthman that he had drunk wine. So ‘Ali spoke to ‘Uthman regarding him, and he (‘Uthman) said: "Take your cousin and flog him." He (‘Ali) said: "Stand up, O Hasan." He (Hasan) said: "What do you have to do with this? Let someone else do it." He (‘Ali) said: "Rather, you have become incapable, weak, and feeble. Stand up, O ‘Abdullah ibn Ja‘far, and flog him." And ‘Ali counted. When he completed forty, he said: "That is enough for you" — or "Stop." "The Messenger of Allah, may Allah bless him and grant him peace, flogged forty, and Abu Bakr flogged forty, and ‘Umar completed it to eighty, and all of it is Sunnah."