Hadith 1682
Chapter 30: The time for witr - كتاب قيام الليل وتطوع النهار
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ نَافِعٍ، أَنَّ ابْنَ عُمَرَ، قَالَ مَنْ صَلَّى مِنَ اللَّيْلِ فَلْيَجْعَلْ آخِرَ صَلاَتِهِ وَتْرًا فَإِنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يَأْمُرُ بِذَلِكَ .
It was narrated that Ibn 'Umar said:
"Whoever prays during the night, let him make the last of his prayers at night witr, because the Messenger of Allah (ﷺ) used to enjoin that."
Reference : Sunan an-Nasa'i 1682
In-book reference : Book 20, Hadith 85