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 (PBUH) said: 'Prayer at night is two by two, and witr is one rak'ah.'" Translation Not Available
Book Ref: Sunan An Nisai Book 20 Hadith 1693
Web Ref: Sunan An Nisai Vol 2 Book 20 Hadith 1694