Mishkat al-Masabih #2685

Ibn ‘Umar said that a man told the Prophet that he was being deceived in business transactions, and he replied, "When you make a bargain say, 'There is no attempt to deceive'*.” Then the man made a habit of saying that. (Bukhari and Muslim.) * He was to say this to protect himself because he was not expert enough to detect defects in what he was buying. Some hold that one who says this has the right to cancel the transaction if any defect is discovered within three days. Some say this applied only to the man who spoke to the Prophet; others say it applies to anyone.