Musnad Abu Ya'la al-Mawsili 153:180

A man came to Abdullah bin Umar and said, "How should I perform the Witr prayer?" He said, "Perform Witr with one rak'ah." The man said, "I fear that the people will say it is the 'Batira' (the cut-off/incomplete prayer)." He said, "Do you desire the Sunnah of Allah and the Sunnah of His Messenger? This is the Sunnah of Allah and the Sunnah of His Messenger."