Musnad Ahmad 27:128

Sufyan narrated to us, Muslim ibn Abi Maryam narrated to me, from Ali ibn Abd al-Rahman al-Mu'awi, who said: I prayed beside Ibn Umar, and I turned over the pebbles. He said: "Do not turn over the pebbles, for it is from Satan. But [do] as I saw the Messenger of Allah, may Allah bless him and grant him peace, do; he used to move them like this." Abu Abdullah said: "Meaning, a wiping motion."