Sahih Muslim Book 4, Hadith Number 1653.

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: The Messenger of Allah (may peace be upon him) was asked about the prayer which was most excellent. He said: That in which the standing is longer. (This Hadith is narrated by another chain of transmitters too.)