Musnad Ahmad 789:50

Abu Kamil narrated to us, Hammad bin Zayd narrated to us, from Jamil bin Murrah, from Abu al-Wadi’, who said: We were on a journey, and Abu Barzah was with us. Abu Barzah said: Indeed, 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."