Musnad Ahmad 937:198

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Shamir bin 'Atiyyah, from his elders, from Abu Dharr, who said: I said: "O Messenger of Allah, advise me." He said: "If you commit a bad deed, follow it with a good deed; it will erase it." I said: "O Messenger of Allah, is 'La ilaha illa Allah' (there is no god but Allah) among the good deeds?" He said: "It is the best of the good deeds."