Sunan An-Nasa'i

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)