Musnad Ahmad 27:1309

'Affan narrated to us, Hammam narrated to us, Qatadah narrated to us, from 'Abdullah ibn Shaqiq al-'Uqayli, from Ibn 'Umar, that a man from the people of the desert asked the Messenger of Allah, peace and blessings of Allah be upon him, about the night prayer. He said with his two fingers: "Two by two, and the Witr is one rak'ah from the end of the night."