Numbers 18:17
But the firstling of a cow, 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 unto the Lord.
Cross-reference
Deuteronomy 15:19–22
Parallel
Deuteronomy 15:19-22 gives instructions for firstborn clean animals, expanding on the rule here that they are holy and not redeemed.
Leviticus 27:26
Parallel
In Leviticus 27:26, firstborn animals already belong to Lord and can't be dedicated — same rule as here.
Genesis 4:4
Related theme
In Genesis 4:4, Abel offered firstborn of flock with fat — similar theme of firstborn sacrifice accepted by God.