Imam Malik's Muwatta, Chapter: Prayer

Yahya related to me from Malik from Abd ar-Rahman ibn al-Qasim that Abdullah ibn Umar told him that he used to see Abdullah ibn Umar cross his legs in the sitting position of the prayer.He said, "So I did the same, and I was young at the time. Abdullah ibn Umar forbade me and said, 'The sunna of the prayer is that you keep your right foot vertical and lay your left foot down.' I said to him, 'But you do the same (as I did).' He said, 'My feet do not support me.' "