Sahih Al Bukhari Hadith Number 1140

Narrated `Aisha: The Prophet (peace be upon him) used to offer thirteen rak`at of the night prayer and that included the witr and two rak`at (Sunna) of the Fajr prayer.

19  Prayer at Night (Tahajjud)
Hadith number 1140
Volume no 2
Chapter: 10 ( How was the Salat of the Prophet (peace be upon him) and how many Rak’a he used to offer at night?)