Positive commandment · Mitzvat Aseh #79 of 613
To sanctify the firstborn of a kosher animal and offer it as a sacrifice
לקדש בכור הבהמה הטהורה ולהקריבו
The verse · Devarim 15:19
כׇּל־הַבְּכוֹר אֲשֶׁר יִוָּלֵד בִּבְקָרְךָ וּבְצֹאנְךָ הַזָּכָר תַּקְדִּישׁ לַה׳ אֱלֹהֶיךָ לֹא תַעֲבֹד בִּבְכֹר שׁוֹרֶךָ וְלֹא תָגֹז בְּכוֹר צֹאנֶךָ
“All the firstling males that come of thy herd and of thy flock thou shalt sanctify to the Lord thy God: thou shalt do no work with the firstling of thy bullock, nor shear the firstling of thy sheep.”
The Rambam’s words
To sanctify the firstborn of a kosher animal and offer it as a sacrifice, as [Deuteronomy 15:19] states: "Every firstling which shall be born...."
לקדש בכור הבהמה הטהורה ולהקריבו, שנאמר: "כל הבכור אשר יולד" (דברים טו,יט).
Why · Sefer HaChinuch #18
Sanctifying the firstborn in the Land of Israel
“It is from the roots of this commandment that G-d, may He be blessed, wanted to make us merit to do a commandment with the beginning of His fruit, in order that all should know that everything is His; and that man has nothing in the world, except that which G-d, may He be blessed, apportions to us, in His kindness.”
Who & when
“And the commandment of the sanctification of the firstborn pure animal is practiced according to the Torah in the Land of Israel alone, at all times; and like the rabbis expounded (Temurah 21b, and see Mishneh Torah, Laws of Firstlings 1:5), “From that which it is written (Deuteronomy 14:23), ‘And you shall eat in front of the Lord, your G-d, the tithes of your new grain and wine and oil, and the firstborn of your herds’ — it compares, etc.””
Where in the Mishneh Torah
Hilchot Firstlings הלכות בכורות