It is narrated from Sayyiduna Hakim bin Hizam (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The buyer and the seller have the option as long as they have not separated. If both of them told the truth and made things clear, they will be blessed; and if they lied and concealed, the blessing of their transaction will be wiped out."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3105