It was narrated from Ibn 'Umar that: a man told the Messenger of Allah that he was always being cheated. The Messenger of Allah said to him: When you make a deal, say: There is no intention of cheating So, whenever the man engages in a deal he says, 'There is no intention of cheating. (Sahih )
It was narrated from Anas that: there was a man whose mental faculties were lacking, and he used to buy and sell. His family came to the Prophet and said: O Prophet of Allah stop him. So the Prophet of Allah called him, and told him not to do that. He said: O Prophet of Allah, I cannot bear to be away from business, He said If you engaged in a deal then say: 'There is no intention to deceive. ' (Sahih )