Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates: We were with the Messenger of Allah (peace and blessings be upon him) in prayer, and a man was turning over the pebbles with his hand. When the Messenger of Allah (peace and blessings be upon him) finished the prayer, he turned towards us and said: "Which of you is the one who was turning over the pebbles?" A man stood up and said: It is me, O Messenger of Allah. He (peace and blessings be upon him) said to him: "That is your share of your prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13170