There is a nice story concerning this I do not know if it is true or not but it has a good moral and since it is not a hadith etc there is inshAllah no harm in telling it.
It is about an Imaam who had moved to a new city he had got a job there as the Imaam of a new masjid. One day he boarded a bus and was travelling through the city. He paid the driver who appeared to be a muslim, and took his change and ticket and found a seat. When the Imaam quickly counted the money he had recieved back he suddenly realised that he had got a penny or so more then he should have. He sat in his seat and thought to himself shall I go and return the money it is the right thing to do, but at the same time Shaytaan whispered to him, it is only a penny it will not make a big difference, do you really think this big bus company will notice that you took a penny from them? Plus its the drivers fault not yours. But then the Imaam realised even if the busdriver nor the company would notice Allah the Most High above the seven heavens would surely notice!
So when it was time for his stop he went by the driver and returned the money and said I think you gave me the wrong change by mistake, the driver smiled at him and said no shaykh I did not, I have heared much about you and that you are our new local Imaam and I wanted to see if you really were a man of God or not.