The 79th mitzvah is that we are commanded to sanctify the firstborn, i.e., to separate them and set them aside for whatever we are obligated to do with them.
The source of this commandment is G‑d's statement1 (exalted be He), "Sanctify to Me every firstborn that opens the womb among the Jewish people; among both man and animal." Scripture dictates that the "animal," referred to includes only cattle, sheep and donkeys. The commandment is repeated regarding the firstborn of kosher species, which is the commandment we are talking about now, in G‑d's statement,2 "You must consecrate to G‑d every firstborn male born among your cattle and sheep."
The law regarding this firstborn kosher animal is that it must be given to the priests, who sacrifice the fat and blood and consume the remaining meat.
The details of this mitzvah have been completely explained in tractate Bechoros.
It is explained at the end of tractate Challah3 that this commandment applies only in Israel. In the words of the Sifri,4 "One might think that a person is obligated to bring his firstborn [animals] from outside Israel to Israel. The Torah therefore says, 'You must eat before G‑d your L‑rd...the [second] tithe of your grain, wine, and oil, as well as the firstborn of your cattle and sheep.'5 [Only] from the place you bring the [second] tithe of your grain do you bring your firstborn. But from outside Israel, from where you do not bring the [second] tithe of your grain, you do not bring [from there] your firstborn."
It has been explained to you that this commandment applies only in Israel, but a firstborn animal born outside Israel is not sacrificed. However, it remains holy until it acquires a blemish and then may be eaten. This applies both when the Temple was standing and as it is now [i.e., after its destruction], just like the [second] tithe of grain.
(Levites are exempt from this commandment.)6
