Sunan an-Nasa'i, Hadith 1693 ⓘ
It was narrated that Ibn Umar said: "The Messenger of Allah said: 'Prayer at night is two by two, and witr is one rak'ah.'"
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا خَالِدُ بْنُ زِيَادٍ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى وَالْوَتْرُ رَكْعَةٌ وَاحِدَةٌ " .
also referenced under: Book 20, Hadith 96 (In-Book) & Volume 2, Book 20, Hadith 1694 (DarusSalam)