Musnad Ahmad 27:386

Mu'adh narrated to us, Ibn 'Awn narrated to us, from Muslim, a freed slave of 'Abd al-Qays. Mu'adh said: Shu'bah used to say: Al-Qari. He said: A man said to Ibn 'Umar: "Do you see the Witr, is it a Sunnah?" He said: "What is a Sunnah? The Messenger of Allah (ﷺ) performed Witr, and the Muslims performed Witr." He said: "No, is it a Sunnah?" He said: "Desist! Do you understand? The Messenger of Allah (ﷺ) performed Witr, and the Muslims performed Witr."