Musnad Abu Ya'la al-Mawsili 3:338

Abu Khaythama narrated to us, Ismail bin Ibrahim narrated to us, from Sa’id bin Abi ‘Aruba, from ‘Abdullah al-Danaj, from Hudayn Abu Sasan: That some people from the people of Kufa rode to ‘Uthman bin ‘Affan and informed him of what had happened regarding the affair of al-Walid, meaning that he was drinking wine. So ‘Ali spoke to him about that. ‘Uthman said to him, "Take your cousin and carry out the prescribed punishment upon him." He said, "Stand up, O Hasan, and flog him." He (Hasan) said, "What do you have to do with this? Let someone else do it." He said, "Rather, you have become weak and feeble. Stand up, O ‘Abdullah bin Ja’far, and flog him." So he began to flog him, and ‘Ali was counting until he reached forty. He then said, "Stop, and let him go. The Messenger of Allah (ﷺ) flogged forty, and Abu Bakr flogged forty, and ‘Umar completed it to eighty. And all of it is Sunnah."