Muwatta Imam Malik (Riwayat Ibn al-Qasim)

It is narrated from Ali bin Abdur Rahman al-Ma'adi (Tabi'i) that Abdullah bin Umar (may Allah be pleased with them both) saw me (during prayer) and I was idly playing with pebbles. When I finished the prayer with salam, he forbade me and said: "Do as the Messenger of Allah (peace and blessings be upon him) used to do." I asked: "What did the Messenger of Allah (peace and blessings be upon him) do in this situation?" He replied: "When the Messenger of Allah (peace and blessings be upon him) sat in prayer, he would place his right palm on his right thigh, close all his fingers, and point with the finger next to the thumb (the index finger), and he would place his left palm on his left thigh."

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.