Musnad Abu Ya'la al-Mawsili 154:138

Abu al-Rabi' al-Zahrani narrated to us, Hammad bin Zayd narrated to us, Anas bin Sirin narrated from Ibn Umar, that the Messenger of Allah (ﷺ) used to pray during the night two by two, and he would perform witr with one rak'ah, and he would pray two rak'ahs before the dawn prayer.