Sahih Muslim Book 4, Hadith Number 1602.

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 pray eleven rak’ahs at night, observing the Witr with a single rak’ah, and when he had finished them, he lay down on his right side, till the Mu’adhdhin came to him and he (the Holy Prophet) then observed two short rak’ahs (of Sunan of the dawn prayer).