Musannaf Ibn Abi Shayba 3:4473

Mu'adh ibn Mu'adh narrated to us, from Ibn 'Awn, from Muslim, the freed slave of 'Abd al-Qays, who said: A man said to Ibn 'Umar, "What do you see? Is the Witr a Sunnah?" He said: He said, "What is 'a Sunnah'? The Messenger of Allah (ﷺ) performed Witr, and the Muslims performed Witr." He said: "Is it not a Sunnah?" He said: "Be quiet! Are you heedless? The Messenger of Allah (ﷺ) performed Witr, and the Muslims performed Witr."