Musnad Ahmad 31:592

Isma'il narrated to us, from al-Jurayri, from Abu Nadra, who said: I asked Ibn Abbas about *sarf* (currency exchange), and he said: "Hand to hand." I said: "Yes." He said: "There is no harm in it." Then I met Abu Sa'id al-Khudri and informed him that I had asked Ibn Abbas about *sarf*, and he said: "There is no harm in it." He (Abu Sa'id) said: "Did he say that? We shall write to him, and he will not give you a fatwa for it." He said: By Allah, some of the young men of the Messenger of Allah (ﷺ) brought dates, and he (the Prophet) disapproved of them. He said: "It is as if this is not from the dates of our land?" He said: "Our dates this year were of a certain quality, so I took these and added some extra." He said: "You have doubled, you have committed *riba* (usury). Do not approach this. If you are in doubt about any of your dates, sell them, then buy the dates you desire."