Ash-Shama'il Al-Muhammadiyah, Book 20, Hadith 121 (English)

The uncle of 'Abbaad bin Tamim, 'Abdullah bin Zayd Radiyallahu 'Anhu reports: "I have seen Rasulullah Sallallahu 'Alayhi Wasallam lying flat on his back in the masjid, with one leg resting on the other.

حَدَّثَنَا سَعِيدُ بْنُ عَبْدِ الرَّحْمَنِ الْمَخْزُومِيُّ، وَغَيْرُ وَاحِدٍ، قَالُوا‏:‏ حَدَّثَنَا سُفْيَانُ، عَنِ الزُّهْرِيِّ، عَنْ عَبَّادِ بْنِ تَمِيمٍ، عَنْ عَمِّهِ، أَنَّهُ رَأَى النَّبِيَّ صلى الله عليه وسلم مُسْتَلْقِيًا فِي الْمَسْجِدِ وَاضِعًا إِحْدَى رِجْلَيْهِ عَلَى الأُخْرَى‏.‏

also referenced under: Book 21, Hadith 128 (In-Book)