Exodus 13:2

2 aConsecrate to me all the firstborn. Whatever is the first to open the womb among the people of Israel, both of man and of beast, is mine.”

Numbers 18:15-17

15 bEverything that opens the womb of all flesh, whether man or beast, which they offer to the Lord, shall be yours. Nevertheless, cthe firstborn of man you shall redeem, and the firstborn of unclean animals you shall redeem. 16And their redemption price d(at a month old you shall redeem them) you shall fix at five shekels
A  shekel was about 2/5 ounce or 11 grams
in silver, according to the shekel of the sanctuary, fwhich is twenty gerahs.
17 gBut the firstborn of a cow, or the firstborn of a sheep, or the firstborn of a goat, you shall not redeem; they are holy. You shall sprinkle their blood on the altar and shall burn their fat as a food offering, with a pleasing aroma to the Lord.
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