He who prayed at night should make Witr the end of his prayer, for the Messenger of Allah ﷺ ordered this. (Using translation from Muslim 751a)
«مَنْ صَلَّى مِنَ اللَّيْلِ فَلْيَجْعَلْ آخِرَ صَلَاتِهِ وِتْرًا فَإِنَّ رَسُولَ اللهِ ﷺ كَانَ يَأْمُرُ بِذَلِكَ»