It is narrated from Sayyiduna Samurah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "The buyer and the seller both have the option (to cancel the sale) as long as they have not separated, unless their sale is made with a stipulated option."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6803