Sayyiduna 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." Hammam says: I found in my book that they give the option three times. "If both of them speak the truth and make things clear, their trade will be blessed; if they lie and conceal, it may be that they make a profit, but the blessing will be wiped out."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10437