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