The Book of Prayer - Travellers
6/17. Chapter:
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
Sahih Muslim
وَحَدَّثَنَا زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا جَرِيرٌ، عَنِ الأَعْمَشِ، عَنْ تَمِيمِ بْنِ سَلَمَةَ، عَنْ عُرْوَةَ بْنِ الزُّبَيْرِ، عَنْ عَائِشَةَ، قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُصَلِّي مِنَ اللَّيْلِ فَإِذَا أَوْتَرَ قَالَ " قُومِي فَأَوْتِرِي يَا عَائِشَةُ " .
'A'isha reported: The Messenger of Allah (ﷺ) used to pray in the night and when he observed Witr, he said to me: O 'A'isha, get up and observe Witr.
Reference : Sahih Muslim 744aIn-book reference : Book 6, Hadith 163USC-MSA web (English) reference : Book 4, Hadith 1618