Sahih Muslim Book 4, Hadith Number 1621.

Chapter : Permissibility of observing Nafl (voluntary prayer) standing or sitting and observing some part of it standing or sitting.

Masruq reported on the authority of ‘Aisha that she said that the Messenger Of Allah (may peace be upon him) used to observe the Witr prayer every night, maybe in the early part of night, at midnight and in the latter part, finishing his Witr at dawn.