(20) باب صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى وَالْوِتْرُ رَكْعَةٌ مِنْ آخِرِ اللَّيْلِ
Ibn 'Umar reported Allah's Messenger (ﷺ) as saying:
Witr is a rak'ah at the end of the prayer.
Reference: Sahih Muslim 752 a
In-book reference: Book 6, Hadith 186
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1641