Sahih Muslim 740
Sahih Muslim · Book 6: The Book of Prayer - Travellers · Night prayers and the number of rak`ah offered by the Prophet (saws) at night, and that Witr is one rak`ah, and a one-rak`ah prayer is correct
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، وَأَبُو كُرَيْبٍ قَالاَ حَدَّثَنَا يَحْيَى بْنُ آدَمَ ، حَدَّثَنَا عَمَّارُ بْنُ رُزَيْقٍ ، عَنْ أَبِي إِسْحَاقَ ، عَنِ الأَسْوَدِ ، عَنْ عَائِشَةَ ، قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُصَلِّي مِنَ اللَّيْلِ حَتَّى يَكُونَ آخِرَ صَلاَتِهِ الْوِتْرُ .
'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.