Musnad Ahmad 27:1088

Muhammad narrated to us, Shu'bah narrated to us, from Abu Bishr, I heard Abdullah ibn Shaqiq narrating from Ibn Umar, that a man asked the Prophet, peace and blessings of Allah be upon him, about the Witr? He said: So I walked with that man, and the Messenger of Allah, peace and blessings of Allah be upon him, said: "The prayer of the night is two by two, and the Witr is one rak'ah." Shu'bah said: "He did not say 'from the end of the night'."