Sahih Muslim Book 4, Hadith Number 1618.

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 pray in the night and when he observed Witr, he said to me: O ‘Aisha, get up and observe Witr.