Midrash for Bekhorot 107:17

(Devarim 18:4) "The first (i.e., terumah) of your corn": We are hereby taught that terumah is given only of the choicest. "your corn, your wine, and your oil": We are hereby taught that terumah is not given from one for the other. And just as we find that we do not give terumah from two of these kinds from one for the other, so, we do not give terumah from grains and from greens from one kind for the other.