Sahih Muslim Book 010, Hadith Number 3659.
Chapter : Both the buyers and the sellers have the option to withdraw the transaction before leaving the meeting (where the bargain is struck).
Abdullah b. 'Umar (Allah be pleased with them) reported Allah's Messenger (may peace be upon him) as saying: When two persons enter into a transaction, each one of them has the right to annul it so long as they are not separated, or their transaction gives one another (as a condition) the right of annulling, and if their transaction has the right of annulling it the transaction becomes binding. Ibn Abi Umar made this addition that whenever he (Ibn Umar) entered into a transaction with a person with the intention of not breaking it, he walked a while and then returned to him.