Sahih Muslim 751
Sahih Muslim · Book 6: The Book of Prayer - Travellers · The night prayers are two by two, and Witr is one rakah at the end of the night
Nafi' reported Ibn 'Umar as saying:
وَحَدَّثَنِي هَارُونُ بْنُ عَبْدِ اللَّهِ ، حَدَّثَنَا حَجَّاجُ بْنُ مُحَمَّدٍ ، قَالَ قَالَ ابْنُ جُرَيْجٍ أَخْبَرَنِي نَافِعٌ ، أَنَّ ابْنَ عُمَرَ ، كَانَ يَقُولُ مَنْ صَلَّى مِنَ اللَّيْلِ فَلْيَجْعَلْ آخِرَ صَلاَتِهِ وِتْرًا قَبْلَ الصُّبْحِ كَذَلِكَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَأْمُرُهُمْ .
He who observed the night prayer should make Witr the end of his prayer before dawn. The Messenger of Allah (ﷺ) used to order them thus.