Sahih Al Bukhari – Book of Characteristics Of Prayer

Volumn 001, Book 012, Hadith Number 790. 
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Narated By 'Abdullah bin 'Abdullah RA : I saw 'Abdullah bin 'Umar RA  crossing his legs while sitting in the prayer and I, a mere youngster in those days, did the same. Ibn 'Umar forbade me to do so, and said, "The proper way is to keep the right foot propped up and bend the left in the prayer." I said questioningly, "But you are doing so (crossing the legs)." He said, "My feet cannot bear my weight."