And the 124th prohibition is that we are forbidden from baking the leftover part of the meal-offerings [remaining after the handful is burned on the altar] as chametz.
The source of this prohibition is G‑d's statement1 (exalted be He), "It shall not be baked as chametz. I have given this to them as their portion of My fire-offerings." It is as if the verse says that their portion [i.e., that of the kohanim] — which is the leftover part of the meal-offering — may not be baked as chametz.
One who does bake it as chametz is punished by lashes, as the Mishneh2 explains, "One is punished for baking it."
The details of this mitzvah are explained in the 5th chapter of tractate Menachos.3