Sahih Muslim Book 24, Hadith Number 5237.

Chapter : It Is prohibited to lie down on one’s back and placing of one’s foot upon the other.

Jabir. b. Abdullah reported Allah’s Messenger (may peace be upon him) as saying: Do not walk in one sandal and do not wrap the lower garment round your knees and do not eat with your left hand and do not wrap yourself completely leaving no room for the arms (to draw out) and do not place one of your feet upon the other while lying on your back.