Sahih Muslim 8888
In-book no 1611حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنِ ابْنِ شِهَابٍ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يُصَلِّي بِاللَّيْلِ إِحْدَى عَشْرَةَ رَكْعَةً يُوتِرُ مِنْهَا بِوَاحِدَةٍ فَإِذَا فَرَغَ مِنْهَا اضْطَجَعَ عَلَى شِقِّهِ الأَيْمَنِ حَتَّى يَأْتِيَهُ الْمُؤَذِّنُ فَيُصَلِّي رَكْعَتَيْنِ خَفِيفَتَيْنِ .
'A'isha reported that the Messenger of Allah (ﷺ) used to pray eleven rak'ahs at night, observing the Witr with a single rak'ah, and when he had finished them, he lay down on his right side, till the Mu'adhdhin came to him and he (the Holy Prophet) then observed two short rak'ahs (of Sunan of the dawn prayer).