Musannaf Ibn Abi Shayba 13:2606

Abu Bakr narrated to us, he said: Yazid ibn Harun narrated to us, from Sa'id, from Qatadah, from Salih Abu al-Khalil, from Abdullah ibn al-Harith, from Hakim ibn Hizam, that the Prophet (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."