Yahya related to me from Malik from Abd ar-Rahman ibn al-Qasimthat Abdullah ibn Umar told him that he used to see Abdullah ibn Umarcross his legs in the sitting position of the prayer.He said, "So Idid the same, and I was young at the time. Abdullah ibn Umar forbademe and said, 'The sunna of the prayer is that you keep your right footvertical and lay your left foot down.' I said to him, 'But you do thesame (as I did).' He said, 'My feet do not support me.' "
App reference: Book 3, Hadith 54
USC-MSA web (English) reference: Book 3, Hadith 54
Arabic reference: Book 3, Hadith 201