Maliks Muwatta Book 3, Hadith Number 51.

Section : Behaviour in the Sitting in the Prayer.

Yahya related to me from Malik from Muslim ibn Abi Maryam that AIi ibn Abd ar-Rahman al-Muawi said, “Abdullah ibn Umar saw me playing with some small pebbles in the prayer. When I finished he forbade me, saying, ‘Do as the Messenger of Allah, may Allah bless him and grant him peace, did.’ I said, ‘What did the Messenger of Allah, may Allah bless him and grant him peace, do?’ He said, ‘When he sat in the prayer, he placed his right hand on his right thigh and he closed his fist and pointed his index finger, and he placed his left hand on his left thigh. That is what he used to do.'”