Musnad Ahmad Hadith 13276

It is narrated from Anas (may Allah be pleased with him) that during the blessed era of the Prophet (ﷺ), there was a man "who had some weakness in his intellect" and used to buy and sell (and would be deceived). His family members came to the service of the Prophet (ﷺ) and said, O Messenger of Allah (ﷺ)! Impose a restriction on so-and-so regarding buying and selling because his intellect is weak. The Prophet (ﷺ) called him and forbade him from buying and selling. He said, O Messenger of Allah! I cannot refrain from this work. The Prophet (ﷺ) said, If you cannot leave buying and selling, then when making a transaction, say: "There is no deception in this transaction."