(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 night prayer. (ES)
Reference (ES): Sahih Muslim 752 b (ES)
In-book reference (ES): Book 6, Hadith 187 (ES)
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1642 (ES)