Negative commandment · Lo Ta’aseh #108 of 613
Not to redeem a firstling kosher animal
שלא לפדות בכור בהמה טהורה
The verse · Bamidbar 18:17
אַךְ בְּכוֹר־שׁוֹר אוֹ־בְכוֹר כֶּשֶׂב אוֹ־בְכוֹר עֵז לֹא תִפְדֶּה קֹדֶשׁ הֵם אֶת־דָּמָם תִּזְרֹק עַל־הַמִּזְבֵּחַ וְאֶת־חֶלְבָּם תַּקְטִיר אִשֶּׁה לְרֵיחַ נִיחֹחַ לַה׳
“But the firstling of an ox, or the firstling of a sheep, or the firstling of a goat, thou shalt not redeem; they are holy: thou shalt sprinkle their blood upon the altar, and shalt burn their fat for an offering made by fire, for a sweet savour to the Lord.”
The Rambam’s words
Not to redeem a firstling kosher animal, as [Numbers 18:17] states: "Nevertheless, a firstling ox... do not redeem."
שלא לפדות בכור בהמה טהורה, שנאמר "אך בכור שור . . . לא תפדה" (במדבר יח,יז).
Who & when
“And this commandment is practiced in the Land of Israel, like the commandment of sanctifying the firstborn pure animals, which is only practiced in the Land of Israel, according to some commentators (see Beit Yosef, Yoreh Deah 306) — both when the Temple [is standing] and when the Temple [is not standing] — as it is stated (Deuteronomy 14:23), “And you shall eat before the Lord, Your G-d, the tithes of your grain, etc. and the firstborn of your cattle and your sheep.””
Where in the Mishneh Torah
Hilchot Firstlings הלכות בכורות