He saw the Prophet ﷺ lying on his back in the mosque with one leg crossed over the other.
حدثنا سعيد بن عبد الرحمن المخزومي وغير واحد قالوا حدثنا سفيان بن عيينة عن الزهري
أنه رأى النبي ﷺ مستلقيا في المسجد واضعا إحدى رجليه على الأخرى
Agreed upon [by Bukhari and Muslim]. Bukhari narrated it in the Book of Seeking Permission, Chapter on Lying on One's Back, hadith no. 6287, and Muslim, hadith no. 2100, from the hadith of Sufyan bin Uyayna, through this same chain.