Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates from the Messenger of Allah (peace and blessings be upon him) that he (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." Nafi' (may Allah have mercy on him) says that when Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) bought something from someone and wanted to make the sale binding, he would part from his companion.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10432