Deuteronomy 12:17 "You may not eat within your gates...the first- born of your herds or flocks"

The first-born animals must be redeemed or offered as sacrifices in the Beit Hamikdash (See positive Mitzvah 79).

The priest is entitled to portions of the meat that are not burnt.

A non-priest is not given a portion of this sacrifice.

This Negative Mitzvah applies only to a "perfect" first born animal, one that has no blemishes and is fit to be sacrificed.

This Negative Mitzvah contains two distinct laws.

The first is that a priest may not eat his portion of this first- born sacrifice outside of Jerusalem. The second is that a non- priest is forbidden to eat any of this meat at all.