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 إسحق بن موسى الأنصاري حدثنا معن بن عيسى حدثنا مالك عن ابن شهاب عن عروة
أن رسول الله ﷺ كان يصلي من الليل إحدى عشرة ركعة يوتر منها بواحدة فإذا فرغ منها اضطجع على شقه الأيمن
[Its chain of transmission is daif, Zuhri narrated with "an" without specifying direct hearing]. Muslim also transmitted it in "The Book of the Traveler's Prayer," in the chapter "The Night Prayer and the Number of Units the Prophet ﷺ Prayed at Night... etc.," hadith no. 121/736, from the hadith of Malik with different wording, and it appears in Muwatta 1/120 (Yahya's version). See the hadith that follows, where lying down comes after the two units, and that is the correct version.