מדרש על משלי 14:9

2) I might think that it is a mitzvah for a Cohein to take the fistful, but that if a non-Cohein does so it is also kasher; it is, therefore, written (Mishlei 14:9): "And the Cohein shall lift from the meal-offering its remembrance" (i.e., the fistful") — to stipulate that if a non-Cohein does so it is pasul.