إذا تبايع الرجلان فكل واحد منهما بالخيار ما لم يتفرقا وكانا جميعا أو يخير أحدهما الآخر فإن خير أحدهما الآخر فتبايعا على ذلك فقد وجب البيع وإن تفرقا بعد أن تبايعا ولم يترك واحد منهما البيع فقد وجب البيع
It was narrated from ‘Abdullah bin ‘Umar that the Messenger of Allah said:
“When two men enter into a transaction, each of them has the choice (of annulling it) so long as they have not yet parted and are still together, or one of them has given the option or choice (to conclude or annul the transaction) to the other. Once he has accepted the terms of the other, then the transaction is binding. If they part after concluding the transaction and neither of them has rescinded the transaction then the transaction is binding.”
This hadith was narrated by al-Bukhari 2112, Muslim 1531, al-Nasai 7/249, Ibn Majah 2181 from the words of Ibn ‘Umar.
Sheikh al-Albani called the hadith authentic. See Sahih al-jami ‘as-saghir 422, Irvaul-galil 1/1310.
 That is, one of them will offer the other to determine the price of the goods verbally, for which he will buy it, as al-‘Iraqi said about it, referring to the most authoritative opinion in accordance with the madhhab of Imam al-Shafi’i. See Sharh Sunan al-Nasai by M. al-Atyubi 34/141.