Muwatta Imam Malik (Riwayat Ibn al-Qasim)

It is narrated from Ubaydullah bin Abdullah bin Umar bin al-Khattab, may Allah have mercy on him, that he used to see that (his father) Abdullah bin Umar, may Allah be pleased with them both, when he sat in prayer (for Tashahhud), he would sit cross-legged. He said that (once) I also did this, and at that time I was a small child, so Sayyiduna Abdullah bin Umar, may Allah be pleased with them both, forbade me and said: The Sunnah of prayer is that you should raise your right foot and spread your left foot. I said to him: But you sit cross-legged? He replied: My feet cannot support me, that is, I am ill.