(20) باب صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى وَالْوِتْرُ رَكْعَةٌ مِنْ آخِرِ اللَّيْلِ
Ibn 'Umar said:
He who prayed at night should make Witr the end of his prayer, for the Messenger of Allah (ﷺ) ordered this.
Reference: Sahih Muslim 751 a
In-book reference: Book 6, Hadith 183
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1638