Musnad Ahmad 25:1549

Bahz narrated to us, Hammam narrated to us, from Qatadah. And 'Affan narrated to us, he said: Hammam narrated to us, from Qatadah. 'Affan said: Qatadah informed us, from Abu Mijlaz, who said: I asked Ibn 'Umar 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." He said: And I asked 'Abdullah ibn 'Abbas, 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."