Musannaf Ibn Abi Shayba 35:112

Al-Fadl ibn Dukayn narrated to us, from Hammad ibn Zayd, from Jamil ibn Murrah, from Abu al-Wadi, from Abu Barzah, who said: The Prophet, may Allah bless him and grant him peace, said: "The two parties to a transaction have the option (to rescind or confirm) as long as they have not separated."