Musnad Ahmad 2:270

Muhammad ibn Ja'far and Hajjaj narrated to us, they said: Shu'bah narrated to us, from Sa'd ibn Ibrahim, he said: I heard 'Ubayd Allah ibn 'Abd Allah ibn 'Utbah narrating from Ibn 'Abbas, from 'Abd al-Rahman ibn 'Awf, he said: 'Umar ibn al-Khattab performed Hajj, and he intended to deliver a sermon to the people. 'Abd al-Rahman ibn 'Awf said: "The rabble of the people have gathered before you, so delay that until you reach Medina." When he arrived in Medina, I drew near to him, close to the pulpit, and I heard him saying: "Indeed, some people say: 'What is the matter with stoning, when there is only flogging in the Book of Allah?' Yet the Messenger of Allah (peace and blessings of Allah be upon him) stoned, and we stoned after him. And were it not that people would say, 'He added to the Book of Allah that which is not in it,' I would have written it down as it was revealed."