Sayyiduna Anas (may Allah be pleased with him) narrates that in the blessed time of the Prophet (peace and blessings be upon him), there was a man who used to buy and sell, but he had some mental weakness. Once, his family came to the Prophet (peace and blessings be upon him) and said: "O Messenger of Allah! Please stop so-and-so from making transactions, because he makes deals but he has some mental weakness." The Prophet (peace and blessings be upon him) called him and forbade him from making transactions. He said: "I cannot manage without buying and selling." So the Prophet (peace and blessings be upon him) said: "If you must buy and sell, then say: 'This is in exchange for that, and there will be no cheating.'"