Sahih Bukhari
Prayer at Night (Tahajjud)
Volume: 002 / Book Number: 021 / Hadith Number: 240
Narrated Masruq:
I asked Aisha about the night prayer of Allah's Apostle and she said, "It was seven, nine or eleven Rakat besides the two Rakat of the Fajr prayer (i.e. Sunna). "