Musnad Ahmad 27:1798

Jarir narrated to us, from Mansur, from Habib, from Tawus, who said: A man said to Ibn Umar, "Abu Hurayrah claims that the Witr (prayer) is not mandatory." He said: A man asked the Messenger of Allah, peace and blessings of Allah be upon him, about the night prayer. He said: "The night prayer is two by two. So if you fear the morning, then perform Witr with one."