Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 115
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 115, with English translation by Maimonides (Rambam).
שלא ישחוט הפסח והחמץ קיים, שנאמר "לא תשחט על חמץ, דם זבחי" (שמות לד,כה).
Not to slaughter the Paschal sacrifice while one possesses chametz, as [Exodus 23:18] states: "Do not sacrifice the blood of My offering in the presence of chametz."