Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 144
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 144, with English translation by Maimonides (Rambam).
שלא לאכול בכור תמים חוץ לירושלים, שנאמר "לא תוכל . . . ובכורות" (דברים יב,יז).
Not to eat an unblemished firstling animal outside of Jerusalem, as [Deuteronomy, ibid.] states: "You may not eat [within your gates]... and the firstlings [of your cattle and flocks]...."