Musnad Ahmad 28:245

Hammad ibn Mas'adah narrated to us, from Ibn 'Ajlan, from 'Amr ibn Shu'ayb, from his father, from his grandfather, that the Prophet (peace and blessings of Allah be upon him) said: "The seller and the buyer have the option [to rescind the contract] as long as they have not separated, unless it is a transaction with an option [stipulated]. And it is not lawful for him to separate from him out of fear that he might ask him to cancel the sale."