'Amr b. Shu'aib, on his father's authority, said that his grandfather reported God’s Messenger as saying, “Both parties in a business transaction have a right to annul it so long as they have not separated unless it is a bargain with the right to annul it attached to it; and one has not the right to separate from the other for fear that he may demand that the bargain be rescinded.”
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.
