Sunan an-Nasa'i 4485

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)