Sahih Muslim Book 4, Hadith Number 1650.

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: If anyone is afraid that he may not get up in the latter part of the night, he should observe Witr in the first part of it; and if anyone is eager to get up in the last part of it, he should observe Witr at the end of the night, for prayer at the end of the night is witnessed (by the angels) and that is preferable.