Sunan an-Nasai 16434
In-book no 1698أَخْبَرَنَا 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.'"