source of pic : google
Narrated 'Abdullah bin 'Abdullah (Radi-Allahu 'anhu):
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."
Bukhari Vol. 1 : No. 790