(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 observe Witr every night, and he would (at times) complete his Witr at the end of the night.
Reference: Sahih Muslim 745 c
In-book reference: Book 6, Hadith 167
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1622