Sahih Muslim 745
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
حَدَّثَنِي عَلِيُّ بْنُ حُجْرٍ ، حَدَّثَنَا حَسَّانُ، - قَاضِي كِرْمَانَ - عَنْ سَعِيدِ بْنِ مَسْرُوقٍ ، عَنْ أَبِي الضُّحَى ، عَنْ مَسْرُوقٍ ، عَنْ عَائِشَةَ ، قَالَتْ كُلَّ اللَّيْلِ قَدْ أَوْتَرَ رَسُولُ اللَّهِ صلى الله عليه وسلم فَانْتَهَى وِتْرُهُ إِلَى آخِرِ اللَّيْلِ .
'A'isha reported that the Messenger of Allah (ﷺ) used to observe Witr every night, and he would (at times) complete his Witr at the end of the night.