Sahih Muslim Book 4, Hadith Number 1655.

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 reported: I heard the Apostle of Allah (may peace be upon him) as saying: There is an hour during the night in which no Muslim bondman will ask Allah for good in this world and the next but He will grant it to him.