سُنَنُ النَّسَائِيِّ
Sunan an-Nasa’i 1693
The Book of Qiyam Al-Lail (The Night Prayer) and Voluntary Prayers During the Day
How to pray witr with 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 .
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.'"
Sunan an-Nasa'i, 1693Sahih (Darussalam)