Sahih Muslim Book 4, Hadith Number 1619.

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

‘Aisha reported that the Messenger of Allah (may peace be upon him) used to offer prayer at night while she lay in front of him, and when the Witr prayer was yet to be observed, he would awaken her and she observed Witr.