Sunan Ibn Majah, Chapter: The Chapters on Rulings

there was a man at the time of the Messenger of Allah (saw) whose mental faculties were lacking, and he used to buy and sell. His family came to the Prophet (saw) and said, "O Messenger of Allah, stop him." So The Prophet (saw) called him, and told him not to do that. He said: "O Messenger of Allah (saw), I cannot bear to be away from business." He said, "If you engage in a transaction then say: "Take it (i.e., the goods) and don't cheat (me).' "