Musnad Ahmad 27:200

Yahya narrated to us, from Ubaydullah, from Nafi’, from Ibn Umar, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade al-muzabanah." And al-muzabanah is: [the exchange of] dates for dates by measure, and grapes for raisins by measure, and wheat for standing crops by measure.