Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1611
It is reported on the authority of 'A'isha 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.