Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 124
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 124, with English translation by Maimonides (Rambam).
שלא לעשות שיירי מנחות חמץ, שנאמר "לא תיאפה חמץ, חלקם" (ויקרא ו,י).
Not to allow any of the remaining portions of the meal offerings to leaven, as [Leviticus 6:10] states: "It shall not be baked as leaven. Their portion...."