Musnad Ahmad 31:8

Mu'tamir narrated to us, from his father, who said: Abu Nadra informed me, from Abu Sa'id, that the owner of the dates came to the Messenger of Allah, may Allah bless him and grant him peace, with dates, and he disapproved of them. He said: "Where did you get this from?" He said: "We bought a sa' (measure) for two sa's of our dates." The Messenger of Allah, may Allah bless him and grant him peace, said: "You have committed usury (riba)."