It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "The night prayer is two rak'ahs at a time; then, when you fear the arrival of dawn, make witr with one rak'ah."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13127