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 )