Musnad Ahmad 1017:85

Al-Hasan ibn Yahya narrated to us, Al-Fadl ibn Musa narrated to us, from Ubaydullah al-Ataki, from Abdullah ibn Buraydah, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Witr is a duty; so whoever does not perform Witr is not of us." He said it three times.