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) by two. Then when you wish to finish (the prayer), pray one rak'ah, which will make what you have prayed odd (witr) for you."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13040