It was narrated from 'Abdullah bin 'Umar that the Messenger of Allah said:
"When two men enter into a transaction, each of them has the choice (of annulling it) so long as they have not yet parted and are still together, or one of them has given the option or choice to the other. Once he has accepted the terms of the other, then the transaction is binding. If they part after concluding the transaction and neither of them has rescinded the transaction then the transaction is binding."
English reference: Vol. 3, Book 12, Hadith 2181
Arabic reference: Book 12, Hadith 2265