(85) باب الاِسْتِلْقَاءِ فِي الْمَسْجِدِ وَمَدِّ الرِّجْلِ
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: Sahih al-Bukhari 475
In-book reference: Book 8, Hadith 123
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 8, Hadith 464