Sunan An-Nasa'i

It was narrated from Ayyub, from Nafi 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 or chosen to conclude the transaction."" Or perhaps Nafi said: ""Or one of them has said to the other: 'Decide! ""(Sahih)