Musnad Abu Ya'la al-Mawsili 134:8

Al-Abbas ibn al-Walid al-Narsi narrated to us, Yusuf ibn Khalid narrated to us, from al-A'mash, from Anas ibn Malik: That the Messenger of Allah (peace be upon him) saw a man moving pebbles while he was in prayer. When he finished, he said to the man: "It is your share of your prayer."