Sahih Muslim Book 4, Hadith Number 1654.

Chapter : He who fears that he would not be able to observe (Tahajjud) prayer at the end of night should observe Witr in the first part of it.

Jabir said he heard Allah’s Messenger (may peace be upon him) say: There is an hour during the night in which no Muslim individual will ask Allah for good in this world and the next without His giving it to him; and that applies to every night.