'Abdullãh ibne-Mas'úd (Ra) narrates that a man asked: O Rasúlullãh! How can I know when I do good and when I do bad? Nabí (saw) replied: When you hear your neighbours say verily you have done good, then indeed you have done good; and when you hear them say verily you have done bad, indeed you have done bad.

(Tabarãni, Majma-'uz-Zawãid)