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 (صلی اللہ علیہ وسلم) and said: "O Prophet of Allah stop him." So the Prophet (صلی اللہ علیProphet of told him not to do that. He said: "O Prophet (صلی اللہ علیہ وسلم) of Allah, I cProphet of ess," He said "If you engaged in a deal then say: There is no intention to deceive. " (Sahih)