Volume 9, Book 86, Number 94:
Narrated 'Abdullah bin 'Umar:
A man mentioned to the Prophet that he had always been cheated in bargains. The Prophet said, "Whenever you do bargain, say, 'No cheating.'"
Ok, I have no idea how uttering the words "no cheating" is going to stop someone from hustling you.
I went to Chinatown the other day, and I told the Chinese men, "no cheating." Instantaneoulsy, they lowered the prices of their goods to cost, and, oh my... I saved a bundle. I then went to the GAP and uttered this line. Lo and behold, the prices were cut by half.
That is the most clever line I ever heard in my life.
(The above events are sarcastic...)