Musnad Ahmad 937:159

'Abd al-Razzaq narrated to us, Ma'mar and 'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from Abu al-Ahwas, from Abu Dharr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you stands for prayer, mercy faces him, so do not move the pebbles."