(10) باب كَيْفَ كَانَ صَلاَةُ النَّبِيِّ صلى الله عليه وسلم وَكَمْ كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُصَلِّي مِنَ اللَّيْلِ
Narrated Masruq:
I asked Aisha about the night prayer of Allah's Messenger (ﷺ) and she said, "It was seven, nine or eleven rak`at besides the two rak`at of the Fajr prayer (i.e. Sunna). "
Reference: Sahih al-Bukhari 1139
In-book reference: Book 19, Hadith 20
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 21, Hadith 240