Sahih al-Bukhari 1139
Sahih al-Bukhari · Book 19: Prayer at Night (Tahajjud) · How was the Salat of the Prophet (pbuh) and how many Rak'a he used to offer at night?
Narrated Masruq:
حَدَّثَنَا إِسْحَاقُ ، قَالَ حَدَّثَنَا عُبَيْدُ اللَّهِ ، قَالَ أَخْبَرَنَا إِسْرَائِيلُ ، عَنْ أَبِي حَصِينٍ ، عَنْ يَحْيَى بْنِ وَثَّابٍ ، عَنْ مَسْرُوقٍ ، قَالَ سَأَلْتُ عَائِشَةَ ـ رضى الله عنها ـ عَنْ صَلاَةِ رَسُولِ اللَّهِ صلى الله عليه وسلم بِاللَّيْلِ. فَقَالَتْ سَبْعٌ وَتِسْعٌ وَإِحْدَى عَشْرَةَ سِوَى رَكْعَتَىِ الْفَجْرِ.
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). "