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 b

Abu Salama asked 'A'isha about the prayer of the Messenger of Allah (ﷺ) She said:

He observed thirteen rak'ahs (in the night prayer). He observed eight rak'ahs and would then observe Witr and then observe two rak'ahs sitting, and when he wanted to bow he stood up and then bowed down, and then observed two rak'ahs in between the Adhan and lqama of the dawn prayer.
وَحَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا ابْنُ أَبِي عَدِيٍّ، حَدَّثَنَا هِشَامٌ، عَنْ يَحْيَى، عَنْ أَبِي سَلَمَةَ، قَالَ سَأَلْتُ عَائِشَةَ عَنْ صَلاَةِ، رَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَتْ كَانَ يُصَلِّي ثَلاَثَ عَشْرَةَ رَكْعَةً يُصَلِّي ثَمَانَ رَكَعَاتٍ ثُمَّ يُوتِرُ ثُمَّ يُصَلِّي رَكْعَتَيْنِ وَهُوَ جَالِسٌ فَإِذَا أَرَادَ أَنْ يَرْكَعَ قَامَ فَرَكَعَ ثُمَّ يُصَلِّي رَكْعَتَيْنِ بَيْنَ النِّدَاءِ وَالإِقَامَةِ مِنْ صَلاَةِ الصُّبْحِ ‏.‏
Reference : Sahih Muslim 738b
In-book reference : Book 6, Hadith 153
USC-MSA web (English) reference : Book 4, Hadith 1608
  (deprecated numbering scheme)