Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) said: "The two parties to a sale have the option as long as they have not separated, or (unless) it is a sale with an option." And sometimes Nafi' (may Allah have mercy on him) said: Or one of them says to the other: You have the option. [Sahih: Muslim 1531]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10455