Sahih Al Bukhari Hadith Number 2112

Narrated Ibn `Umar: Allah’s Messenger (peace be upon him) said, “Both the buyer and the seller have the option of canceling or confirming the bargain, as long as they are still together; and unless they separate or one of them gives the other the option of keeping or returning the things and a decision is concluded then, in which case the bargain is considered final. If they separate after the bargain and none of them has rejected it, then the bargain is rendered final.”