Sunan an-Nasa'i, Book 20, Hadith 85 (In-Book) ⓘ
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 used to enjoin that."
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 .
also referenced under: Hadith 1682 (Reference) & Volume 2, Book 20, Hadith 1683 (DarusSalam)