Musannaf Ibn Abi Shayba 35:110

Yazid narrated to us, from Shu'bah, from Qatadah, from Salih Abu al-Khalil, from Abdullah ibn al-Harith, from Hakim ibn Hizam, that 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."