Musannaf Ibn Abi Shayba 13:2613

Abu Bakr narrated to us, he said: Ibn Abi Za’idah narrated to us, from Muhammad ibn Ishaq, from Nafi’, from Ibn Umar, who said: "The two parties to a sale have the option (to rescind) as long as they have not separated." And Ibn Umar, when he would sell, would walk away in order to make the sale binding.