Musnad Ahmad 937:263

Yazid narrated to us, Ibn Abi Dhi'b informed us, 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 let him not touch the pebbles nor move them."