Musannaf Ibn Abi Shayba 3:4486

Zayd ibn al-Hubab narrated to us, he said: Abu al-Munib narrated to us, from Abdullah ibn Buraydah, from his father, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Witr is a right (or duty). So whoever does not perform Witr, he is not of us."