He saw the Messenger of Allah ﷺ lying on his back in the mosque with one leg crossed over the other.
أخبرنا قتيبة عن مالك عن ابن شهاب
أنه رأى رسول الله ﷺ مستلقيا في المسجد واضعا إحدى رجليه على الأخرى
Bukhari transmitted it in the chapter on prayer, the section on lying down in the masjid and stretching out the leg, hadith no. 475, and Muslim in the chapter on clothing, on the permissibility of lying down and so on, hadith no. 2100, from the hadith of Malik through this same route, and it is found in Muwatta (Yahya's version), 1/173, and in Kubra, hadith no. 800.