Sunan an-Nasa'i, Hadith 1726 ⓘ
It was narrated from Aishah that: The Prophet used to pray eleven rak'ahs at night, of which one was witr, then he would lie down on his right side.
أَخْبَرَنَا إِسْحَاقُ بْنُ مَنْصُورٍ، قَالَ حَدَّثَنَا عَبْدُ الرَّحْمَنِ، قَالَ حَدَّثَنَا مَالِكٌ، عَنِ الزُّهْرِيِّ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ يُصَلِّي مِنَ اللَّيْلِ إِحْدَى عَشْرَةَ رَكْعَةً وَيُوتِرُ مِنْهَا بِوَاحِدَةٍ ثُمَّ يَضْطَجِعُ عَلَى شِقِّهِ الأَيْمَنِ .
also referenced under: Book 20, Hadith 129 (In-Book) & Volume 2, Book 20, Hadith 1727 (DarusSalam)