'A'isha observed that the Messenger of Allah ﷺ used to observe prayer in the night and the last of his (night) prayer was Witr. (Using translation from Muslim 740)
كَانَ رَسُولُ اللهِ ﷺ يُصَلِّي مِنَ اللَّيْلِ حَتَّى يَكُونَ آخِرَ صَلَاتِهِ الْوَتْرُ