Sahih Muslim 744
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
وَحَدَّثَنِي هَارُونُ بْنُ سَعِيدٍ الأَيْلِيُّ ، حَدَّثَنَا ابْنُ وَهْبٍ ، أَخْبَرَنِي سُلَيْمَانُ بْنُ بِلاَلٍ ، عَنْ رَبِيعَةَ بْنِ أَبِي عَبْدِ الرَّحْمَنِ ، عَنِ الْقَاسِمِ بْنِ مُحَمَّدٍ ، عَنْ عَائِشَةَ ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يُصَلِّي صَلاَتَهُ بِاللَّيْلِ وَهِيَ مُعْتَرِضَةٌ بَيْنَ يَدَيْهِ فَإِذَا بَقِيَ الْوِتْرُ أَيْقَظَهَا فَأَوْتَرَتْ .
'A'isha reported that the Messenger of Allah (ﷺ) used to offer prayer at night while she lay in front of him, and when the Witr prayer was yet to be observed, he would awaken her and she observed Witr.