Musnad Ahmad 5:663

Yazid ibn Harun narrated to us, Sa'id ibn Abi 'Arubah informed us, from 'Abdullah al-Danaj, from Hudayn ibn al-Mundhir ibn al-Harith ibn Wa'lah: That al-Walid ibn 'Uqbah led the people in the morning prayer [performing] four [rak'ahs], then he turned to them and said: "Shall I add more for you?" This was reported to 'Uthman, so he ordered that he be flogged. 'Ali said to al-Hasan ibn 'Ali: "Stand up, O Hasan, and flog him." He [al-Hasan] said: "And what do you have to do with that?" 'Ali said: "Rather, you have become weak and feeble. Stand up, O 'Abdullah ibn Ja'far, and flog him." So 'Abdullah ibn Ja'far stood up and flogged him, while 'Ali was counting. When he reached forty, he said to him: "Stop." Then he said: "The Messenger of Allah, peace and blessings of Allah be upon him, flogged [people] for wine [consumption] forty [lashes], and Abu Bakr flogged forty, and 'Umar [did so] for a portion of his caliphate, then 'Umar completed it to eighty, and all of it is Sunnah."