(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. (ID)
Reference (ID): Sahih Muslim 752 a (ID)
In-book reference (ID): Book 6, Hadith 186 (ID)
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1641 (ID)