Hadith 4710
وَعَنْهُ أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «لَا يستلقين أحدكُم ثمَّ يضع رجلَيْهِ على الْأُخْرَى» . رَوَاهُ مُسلم
It is narrated from Jabir (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: "None of you should lie on his back and place one foot over the other." Narrated by Muslim.