Sahih Muslim Book 4, Hadith Number 1651.

Chapter : Night prayer consists of pairs of Rak’ahs and Witr is a Rak’ah at the end of the night.

Jabir reported Allah’s Messenger (may peace be upon him) as saying: He who amongst you is afraid that he may not be able to get up at the end of the night should observe Witr (in the first part) and then sleep, and he who is confident of getting up and praying at night (i.e. Tahajjud prayer) should observe it at the end of it, for the recitation at the end of the night to visited (by angels), and that is excellent.