Musnad Ahmad 937:44

Harun narrated to us, Abdullah ibn Wahb narrated to us, Yunus informed me, from Ibn Shihab, he said: I heard Abu al-Ahwas, the freed slave of Banu Layth, narrating to us in the gathering of Ibn al-Musayyib, and Ibn al-Musayyib was sitting, that he heard Abu Dharr saying: Indeed, the Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you stands for prayer, mercy faces him, so let him not move the pebbles, or let him not touch the pebbles."