Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) 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 before they separate from one another, or else the sale between them is one with an option." When Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) made a sale and wanted the sale to become binding, he would walk a little and then come back.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10431