Musnad Ahmad 937:157

'Abd al-Razzaq narrated to us, Sufyan informed us, from Ibn Abi Layla, from 'Isa, from 'Abd al-Rahman, from Abu Dharr. And Mu'ammal said: Sufyan narrated to us, from Ibn Abi Layla, from his brother, from his father, from Abu Dharr, who said: I asked the Prophet, peace and blessings of Allah be upon him, about everything, until I asked him about smoothing the pebbles. He said: "One [time], or leave it." Mu'ammal said: Regarding leveling the pebbles, or smoothing [them].