Musnad Ahmad 27:1598

Yunus narrated to us, Layth narrated to us, from Nafi’, from ‘Abdullah, from the Messenger of Allah (may Allah bless him and grant him peace), that he used to say: "Do not trade fruits until their ripeness becomes apparent." He forbade the seller and the buyer. And the Messenger of Allah (may Allah bless him and grant him peace) forbade *al-muzabanah*: that one sells the fruit of his garden—if it is date palms, for dates by measure; and if it is a vineyard, that he sells it for raisins by measure; and if it is crops, that he sells it for a known measure. He forbade all of that.