Sahih Muslim Book 4, Hadith Number 1604.

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

‘Aisha reported: The Messenger of Allah (may peace be upon him) used to observe thirteen rak’ahs of the night prayer. Five out of them consisted of Witr, and he did not sit, but at the end (for salutation).