Musnad Abu Ya'la al-Mawsili 154:110

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