Musnad Ahmad 27:969

'Affan narrated to us, Hammad ibn Salamah narrated to us, Ayyub informed us, from Nafi', from Ibn 'Umar, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The two parties to a transaction have the option [to rescind or confirm] as long as they have not separated, or until one of them says to the other: 'Choose.'"