كتاب صلاة المسافرين وقصرها
6  
The Book of Prayer - Travellers
(17)
Chapter: Night prayers and the number of rak`ah offered by the Prophet (saws) at night, and that Witr is one rak`ah, and a one-rak`ah prayer is correct
(17)
باب صَلاَةِ اللَّيْلِ وَعَدَدِ رَكَعَاتِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي اللَّيْلِ وَأَنَّ الْوِتْرَ رَكْعَةٌ وَأَنَّ الرَّكْعَةَ صَلاَةٌ صَحِيحَةٌ
Sahih Muslim 738 d

Abu Salama is reported to have said. I came to 'A'isha. I said:

O mother, inform me about the prayer of the Messenger of Allah (ﷺ). She said: His (night prayer) in Ramadan and (during other months) was thirteen rak'ahs at night including two rak'ahs of fajr.
وَحَدَّثَنَا عَمْرٌو النَّاقِدُ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ عَبْدِ اللَّهِ بْنِ أَبِي لَبِيدٍ، سَمِعَ أَبَا سَلَمَةَ، قَالَ أَتَيْتُ عَائِشَةَ فَقُلْتُ أَىْ أُمَّهْ أَخْبِرِينِي عَنْ صَلاَةِ رَسُولِ اللَّهِ صلى الله عليه وسلم ‏.‏ فَقَالَتْ كَانَتْ صَلاَتُهُ فِي شَهْرِ رَمَضَانَ وَغَيْرِهِ ثَلاَثَ عَشْرَةَ رَكْعَةً بِاللَّيْلِ مِنْهَا رَكْعَتَا الْفَجْرِ ‏.‏
Reference : Sahih Muslim 738d
In-book reference : Book 6, Hadith 155
USC-MSA web (English) reference : Book 4, Hadith 1610
  (deprecated numbering scheme)