Sunan an-Nasa'i 4472

It was narrated from Al-Laith, from Nafi, from Ibn 'Umar that the Messenger of Allah said:"When two men enter into a transaction, each of them has the choice until they separate." On one occasion he said: "So long as they have not separated and one has not told the other to decide. If one tells the other to decide and they agree upon something, then the transaction is binding. If they separate after entering into a transaction and neither of them has canceled the transaction, then the transaction is binding." (Sahih)