Musnad Ahmad 31:88

Abu Mu'awiyah narrated to us, Dawud ibn Abi Hind narrated to us, from Abu Nadrah, who said: I said to Abu Sa'id: "Did you hear from the Messenger of Allah, may Allah bless him and grant him peace, regarding gold for gold and silver for silver?" He said: "I will tell you what I heard from him. A man who owned dates came to him with good dates, and the dates of the Prophet, may Allah bless him and grant him peace, were called 'al-lawn'. The Messenger of Allah, may Allah bless him and grant him peace, said to him: 'Where did you get these good dates from?'" He said: "I took two sa's of our dates and bought one sa' of this with them." He said: The Messenger of Allah, may Allah bless him and grant him peace, said to him: "You have committed usury (riba)?" Then Abu Sa'id said: "So, is date for date usury, or is silver for silver and gold for gold?"