Musannaf Ibn Abi Shayba 35:312

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, "Do you see the Witr? Is it a Sunnah?" He said, "What is its Sunnah? The Prophet, peace and blessings of Allah be upon him, performed Witr, and the Muslims performed Witr." He said, "No, is it a Sunnah?" He said, "Do you not understand? The Prophet, peace and blessings of Allah be upon him, performed Witr, and the Muslims performed Witr."