Sahih Bukhari
Characteristics of Prayer
Volume: 001 / Book Number: 012 / Hadith Number: 790
Narrated 'Abdullah bin 'Abdullah:
I saw 'Abdullah bin 'Umar 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."