Numbers 18:14-17

¹⁴Every devoted thing in Israel will be yours.

¹⁵Everything that opens the womb, all the firstborn which the people offer to Yahweh, both of man and animal, will be yours. Nevertheless, the people must certainly buy back every firstborn son, and they must buy back the firstborn male of unclean animals.

¹⁶Those that are to be bought back by the people must be bought back after becoming one month old. Then the people may buy them back, for the price of five shekels, by the standard weight of the sanctuary shekel, which equals twenty gerahs.

¹⁷But the firstborn of a cow, or the firstborn of a sheep, or the firstborn of a goat—you must not buy back these animals; they are set apart to me. You must sprinkle their blood on the altar and burn their fat as an offering made by fire, an aroma pleasing to Yahweh.