al-Rabi' bin Sulaiman الربيع بن سليمان · The Book of Financial Transactions

"Nafi dictated to me, from Ibn 'Umar who said: The Messenger of Allah said: 'the two parties to a transaction both have the choice so long as they have not separated, unless they have both chosen to conclude the transaction. If they have both chosen to conclude the transaction, then the transactions binding." (Sahih )