Musnad Ahmad

Ali bin Abdur Rahman Mu'awi (may Allah have mercy on him) says that once I had the opportunity to pray next to Sayyiduna Ibn Umar (may Allah be pleased with them both). I began to turn over pebbles, so he forbade me from doing so and said that this is a satanic act. However, it is permissible to do it in the manner I saw the Prophet Muhammad (peace and blessings be upon him) do: he would move them like this. The narrator demonstrated by passing his hand (he would level the place of prostration, but would not keep doing it repeatedly and spoil the prayer).