Hakim bin Hizam (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "The buyer and the seller have the option to cancel the deal as long as they have not separated. If both are truthful and make things clear, their sale will be blessed for them; if both lie and conceal, the blessing will be wiped out of their sale."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10436