Musnad Ahmad 27:681

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Abdullah ibn Dinar, I heard Ibn 'Umar saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Every two parties to a sale, there is no sale between them until they separate, except for the sale of option (khiyar)."