(17) باب صَلاَةِ اللَّيْلِ وَعَدَدِ رَكَعَاتِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي اللَّيْلِ وَأَنَّ الْوِتْرَ رَكْعَةٌ وَأَنَّ الرَّكْعَةَ صَلاَةٌ صَحِيحَةٌ
Masruq reported on the authority of 'A'isha that she said that the Messenger Of Allah (ﷺ) used to observe the Witr prayer every night, maybe in the early part of night, at midnight and in the latter part, finishing his Witr at dawn.
Reference: Sahih Muslim 745 b
In-book reference: Book 6, Hadith 166
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1621