(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 offer prayer at night while she lay in front of him, and when the Witr prayer was yet to be observed, he would awaken her and she observed Witr.
Reference: Sahih Muslim 744 b
In-book reference: Book 6, Hadith 164
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1619