The Book of Qiyam Al-Lail (The Night Prayer) and Voluntary Prayers During the Day
20/35. Chapter:
How to pray witr with one (rak'ah)
Sunan an-Nasai
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 " .
It was narrated that Ibn Umar said: "The Messenger of Allah (ﷺ) said: 'Prayer at night is two by two, and witr is one rak'ah.'"
Reference : Sunan an-Nasa'i 1693In-book reference : Book 20, Hadith 96English translation : Vol. 2, Book 20, Hadith 1694