Sahih Bukhari Volume 003, Book 034, Hadith Number 322.
Narated By Ibn 'Umar : Allah's Apostle said, "The seller and the buyer have the option of cancelling or confirming the deal unless they separate, or one of them says to the other, 'Choose (i.e. decide to cancel or confirm the bargain now)." Perhaps he said, 'Or if it is an optional sale.' " Ibn Umar, Shuraih, Ash-Shabi, Tawus, Ata, and Ibn Abu Mulaika agree upon this judgment.