Sahih Al Bukhari Hadith Number 1139

Narrated Masruq: I asked Aisha about the night prayer of Allah’s Messenger (peace be upon him) and she said, “It was seven, nine or eleven rak`at besides the two rak`at of the Fajr prayer (i.e. Sunna). “

19  Prayer at Night (Tahajjud)
Hadith number 1139
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?)