Night prayers and the number of rak`ah offered by the Prophet (ﷺ) at night, and that Witr is one rak`ah, and a one-rak`ah prayer is correct

Sahih Muslim · Book 6 · Hadith 745 c

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.