Sahih Muslim 738
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 ابْنُ نُمَيْرٍ ، حَدَّثَنَا أَبِي ، حَدَّثَنَا حَنْظَلَةُ ، عَنِ الْقَاسِمِ بْنِ مُحَمَّدٍ ، قَالَ سَمِعْتُ عَائِشَةَ ، تَقُولُ كَانَتْ صَلاَةُ رَسُولِ اللَّهِ صلى الله عليه وسلم مِنَ اللَّيْلِ عَشَرَ رَكَعَاتٍ وَيُوتِرُ بِسَجْدَةٍ وَيَرْكَعُ رَكْعَتَىِ الْفَجْرِ فَتِلْكَ ثَلاَثَ عَشْرَةَ رَكْعَةً .
It is reported on the authority of 'A'isha that the prayer of Allah's Messenger (ﷺ) in the night consisted of ten rak'ahs. He observed a Witr and two rak'ahs (of Sunan) of the dawn prayer, and thus the total comes to thirteen rak'ahs.