Musnad Ahmad 31:110

Hashim narrated to us, Abu Mu'awiyah—meaning Shayban—narrated to us, from Yahya, from Abu Nadrah al-'Awqi, that Abu Sa'id al-Khudri informed him, saying: I asked the Messenger of Allah, peace and blessings of Allah be upon him, about the Witr. He said: "Perform the Witr before the morning."