Musnad Ahmad 27:1546

Hashim narrated to us, Layth narrated to us, Nafi’ narrated to me, from Abdullah ibn Umar, from the Messenger of Allah (may Allah bless him and grant him peace) that he said: "When two men engage in a sale, each of them has the option [to cancel] as long as they have not separated and are still together. Or, one of them may give the other the option. If one of them gives the other the option and they proceed with the sale on that basis, then the sale has become binding. And if they separate after having engaged in the sale, and neither of them has abandoned the sale, then the sale has become binding."