Musnad Ahmad 27:1710

Zayd ibn Yahya al-Dimashqi narrated to us, Abdullah ibn al-Ala narrated to us, I heard Salim ibn Abdullah saying: I heard Abdullah ibn Umar saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The night prayer is two by two. So if you fear the dawn, then perform Witr with one rak'ah, which will make your prayer Witr for you." He said: And Abdullah used to perform Witr with one.