It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "The night prayer is offered two rak'ahs at a time, and the witr is one rak'ah in the last part of the night."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10917