Musnad Ahmad 27:882

I read to Abd al-Rahman: Malik, and Ishaq narrated to us, Malik informed me, from Muslim ibn Abi Maryam, from Ali ibn Abd al-Rahman al-Mu'awi, that he said: Abdullah ibn Umar saw me while I was playing with pebbles during the prayer. When he finished, he forbade me and said: "Do as the Messenger of Allah, may Allah bless him and grant him peace, used to do." I said: "And how did the Messenger of Allah, may Allah bless him and grant him peace, use to do?" He said: "When the Messenger of Allah, may Allah bless him and grant him peace, sat in the prayer, he would place his right palm on his right thigh, clench all his fingers, point with the finger that is next to the thumb, and place his left palm on his left thigh."