The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1602
'A'isha 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).