Al-Muntaqa Ibn al-Jarud

Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates that the Noble Prophet (peace and blessings be upon him) said: "When two men engage in a transaction, both of them have the option (to cancel the sale) as long as they do not separate. This is the case if both remain in the same place or if they grant each other the option (of canceling the sale). If they grant each other the option and make the sale on that condition, then the sale will be concluded. (Similarly) if, after making the sale, both separate and neither of them rejects the sale, then the sale also becomes binding."