Musnad Ahmad 5:63

Isma'il narrated to us, from Sa'id ibn Abi 'Aruba, from 'Abdullah al-Danaj, from Hudayn Abu Sasan al-Raqashi, who said: Some people from the people of Kufa came to 'Uthman and informed him of what had happened regarding the affair of al-Walid—meaning his drinking of wine. So 'Ali spoke to him about that, and he ('Uthman) said: "Take your cousin and carry out the prescribed punishment (hadd) upon him." 'Ali said: "O Hasan, stand up and flog him." He (Hasan) said: "You have no part in this, and let someone other than you take charge of it." He ('Ali) said: "Rather, you have become weak, feeble, and incapable. Stand up, O 'Abdullah ibn Ja'far." So 'Abdullah began to strike him, and 'Ali counted, until he reached forty. Then he said: "Stop"—or he said: "Desist." The Messenger of Allah (peace and blessings of Allah be upon him) flogged forty, and Abu Bakr flogged forty, and 'Umar completed it to eighty, and all of it is Sunnah.