Musnad Ahmad 937:246

Yahya ibn Sa'id narrated to us, from Sufyan, Habib narrated to me, from Maymun ibn Abi Shabib, from Abu Dharr, from the Prophet (peace and blessings of Allah be upon him), who said: "Fear Allah wherever you are, and treat people with good character, and if you commit a bad deed, perform a good deed that wipes it out."