The Messenger of Allah ﷺ used to pray eleven units at night, making one of them the witr. When he finished, he would lie down on his right side.
حدثنا القعنبي عن مالك عن ابن شهاب عن عروة بن الزبير
أن رسول الله ﷺ كان يصلي من الليل إحدى عشرة ركعة يوتر منها بواحدة فإذا فرغ منها اضطجع على شقه الأيمن
Muslim narrated it in the book of the traveler's prayer, chapter on the night prayer and the number of units the Prophet ﷺ prayed at night, etc., hadith 736, from the hadith of Malik through this same route, and it is found in Muwatta Yahya, 1/120.