The Book of the Masjids
8/28. Chapter:
Lying On One's Back In The Masjid
Sunan an-Nasai
أَخْبَرَنَا قُتَيْبَةُ، 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 .
It was narrated from 'Abbad bin Tamim, from his paternal uncle, that he saw the messenger of Allah (ﷺ) lying on his back in the Masjid, placing one leg on top of the other.
Reference : Sunan an-Nasa'i 721In-book reference : Book 8, Hadith 34English translation : Vol. 1, Book 8, Hadith 722