Sunan an-Nasa'i, Volume 2, Book 20, Hadith 1692 (DarusSalam) ⓘ

It was narrated from Ibn 'Umar that: A man from among the people of the desert asked the Messenger of Allah about prayer at night. He said:"(It is) two by two, and Witr is one rak'ah at the end of the night."

أَخْبَرَنَا الْحَسَنُ بْنُ مُحَمَّدٍ، عَنْ عَفَّانَ، قَالَ حَدَّثَنَا هَمَّامٌ، قَالَ حَدَّثَنَا قَتَادَةُ، عَنْ عَبْدِ اللَّهِ بْنِ شَقِيقٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَجُلاً، مِنْ أَهْلِ الْبَادِيَةِ سَأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم عَنْ صَلاَةِ اللَّيْلِ قَالَ ‏ "‏ مَثْنَى مَثْنَى وَالْوَتْرُ رَكْعَةٌ مِنْ آخِرِ اللَّيْلِ ‏" ‏ ‏.‏