Musnad Ahmad 25:977

'Abd al-Samad narrated to us, Hammam narrated to us, Qatadah narrated to us, from Abu Mijlaz, who said: I asked Ibn 'Abbas about the Witr, and he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "A rak'ah from the end of the night." And I asked Ibn 'Umar, and he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "A rak'ah from the end of the night."