رِيَاضُ الصَّالِحِينَ
Riyad as-Salihin 819
The Book of the Etiquette of Sleeping, Lying and Sitting etc
Manners of Lying down on one's back and placing one leg upon the Other
عَنْ عَبْدِ اللَّهِ بْنِ زَيْدٍ رضى اللَّه عَنْهُ أَنَّهُ رَأَى رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ مُسْتَلْقِيًا فِي الْمَسْجِدِ، وَاضِعًا إِحْدَى رِجْلَيْهِ عَلَى الأُخْرَى. ((مُتَّفَقٌ عَلَيْهِ)).
'Abdullah bin Zaid (May Allah be pleased with them) reported: I saw Messenger of Allah (ﷺ) lying down on his back in the mosque, placing one leg on the other. [Al-Bukhari and Muslim].
Riyad as-Salihin, 819