(17) باب صَلاَةِ اللَّيْلِ وَعَدَدِ رَكَعَاتِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي اللَّيْلِ وَأَنَّ الْوِتْرَ رَكْعَةٌ وَأَنَّ الرَّكْعَةَ صَلاَةٌ صَحِيحَةٌ
'A'isha observed that the Messenger of Allah (ﷺ) used to observe prayer in the night and the last of his (night) prayer was Witr.
Reference: Sahih Muslim 740
In-book reference: Book 6, Hadith 158
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1613