Sahih Muslim Book 10, Hadith Number 3656.

Chapter : Both the buyers and the sellers have the option to withdraw the transaction before leaving the meeting (where the bargain is struck).

Ibn ‘Umar (Allah be pleased with them) reported Allah’s Messenger (may peace be upon him) as saying: Both parties in a business transaction have the right to annul it so long as they have not separated; except in transactions which have been made subject to the right of parties to annul them.