Musnad Ahmad 27:972

'Affan narrated to us, Wuhayb narrated to us, Muslim ibn Abi Maryam narrated to us, from 'Ali ibn 'Abd al-Rahman al-Mu'awi, that a man prayed beside Ibn 'Umar and began to fidget with pebbles. He said: "Do not fidget with pebbles, for it is from Satan, but do as the Messenger of Allah (may Allah bless him and grant him peace) used to do." He said: "Like this." And Wuhayb showed us, and 'Affan described it: "He placed his left hand and spread his fingers on his left knee, and he placed his right hand on his right knee, and it was as if he formed a knot and pointed with his index finger."