Volume 1 · Book 8 · Hadith 464
Narrated 'Abbad bin Tamim: that his uncle said, "I saw Allah's Apostle lying flat (on his back) in the mosque with one leg on the other." Narrated Said bin Al-Musaiyab that 'Umar and 'Uthman used to do the same.
— Sahih Bukhari, Volume 1, Book 8: Prayers (Salat), Hadith #464