(20) باب صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى وَالْوِتْرُ رَكْعَةٌ مِنْ آخِرِ اللَّيْلِ
Nafi' reported Ibn 'Umar as saying:
He who observed the night prayer should make Witr the end of his prayer before dawn. The Messenger of Allah (ﷺ) used to order them thus.
Reference: Sahih Muslim 751 c
In-book reference: Book 6, Hadith 185
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1640