Sahih al-Bukhari 475
Sahih al-Bukhari · Book 8: Prayers (Salat) · To lie flat (on the back) in the mosque
Narrated `Abbad bin Tamim:
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ ، عَنْ مَالِكٍ ، عَنِ ابْنِ شِهَابٍ ، عَنْ عَبَّادِ بْنِ تَمِيمٍ ، عَنْ عَمِّهِ ، أَنَّهُ رَأَى رَسُولَ اللَّهِ صلى الله عليه وسلم مُسْتَلْقِيًا فِي الْمَسْجِدِ، وَاضِعًا إِحْدَى رِجْلَيْهِ عَلَى الأُخْرَى. وَعَنِ ابْنِ شِهَابٍ عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ قَالَ كَانَ عُمَرُ وَعُثْمَانُ يَفْعَلاَنِ ذَلِكَ .
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.