Musnad Abu Ya'la al-Mawsili 154:126

Abu Khaythama narrated to us, Abd al-Samad narrated to us, Hammam narrated to us, from Qatadah, 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."