From Sahih Bukhari (Hadith 475):
Narrated `Abbad bin Tamim:that his uncle said, "I saw Allah's Messenger (ﷺ) lying flat (on his back) in the mosque with one leg on the other." Narrated Sa`id bin Al-Musaiyab that `Umar and `Uthman used to do the same
Reference: Bukhari 475