Musnad Ahmad 27:37

Isma'il narrated to us, Ayyub informed us, from Nafi', from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The two parties to a transaction have the option [to rescind] as long as they have not separated, or unless the sale is a sale of option." He said: And Nafi' would sometimes say: "Or one of them says to the other: 'Choose'."