Musnad Ahmad 30:980

Hashim ibn al-Qasim narrated to us, Ayyub—meaning ibn Utbah—narrated to us, Abu Kathir al-Suhaymi narrated to us, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The two parties to a sale have the option [to rescind or confirm] their sale as long as they have not separated, or their sale is one of option."