(17) باب صَلاَةِ اللَّيْلِ وَعَدَدِ رَكَعَاتِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي اللَّيْلِ وَأَنَّ الْوِتْرَ رَكْعَةٌ وَأَنَّ الرَّكْعَةَ صَلاَةٌ صَحِيحَةٌ
'A'isha reported that the Messenger of Allah (ﷺ) used to pray thirteen rak'ahs during the night including the two rak'ahs (Sunan) of the dawn prayer.
Reference: Sahih Muslim 737 c
In-book reference: Book 6, Hadith 151
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1606