Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 191
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 191, with English translation by Maimonides (Rambam).
שלא לאכול כרמל מתבואה חדשה, שנאמר "וכרמל לא תאכלו" (ויקרא כג,יד).
Not to eat fresh grain from newly grown produce before Pesach, as [Leviticus, ibid.] continues: "[Until that day,...] you may not eat... fresh grain."