سنن النسائي

It was narrated that 'Ali bin Abdur-Rahman said: "Ibn Umar saw me playing with the pebbles while praying. When he finished (praying), he told me not to do that and said: 'Do what the Messenger of Allah (ﷺ) used to do.' I said: 'What did he used to do?' He said: 'When he sat during the prayer, he placed his right hand on his thigh and clenched all his fingers, and pointed with the finger that is next to the thumb, and he put his left hand on his left thigh."