Silsilat al-Ahadith al-Sahihah

Sayyiduna Abu Dharr (may Allah be pleased with him) says: I said, "O Messenger of Allah! Advise me." He (peace and blessings be upon him) said: "If you do something bad, then do a good deed after it, so that the good deed will erase (the effect of) the bad deed." I said, "O Messenger of Allah! Is saying 'La ilaha illallah' (There is no god but Allah) also a good deed?" He (peace and blessings be upon him) said: "This is the best of all good deeds."