Sahih Muslim Book 4, Hadith Number 1611.

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

It is reported on the authority of ‘Aisha that the prayer of Allah’s Messenger (may peace be upon him) in the night consisted of ten rak’ahs. He observed a Witr and two rak’ahs (of Sunan) of the dawn prayer, and thus the total comes to thirteen rak’ahs.