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.
The firstling males of the cow, the sheep, and the goat are holy and must not be redeemed. Those three kinds are distinguished from the other firstborn animals, which could be redeemed. Their blood was to be sprinkled upon the altar and their fat burned as an offering made by fire for a sweet savour to the Lord.
The wider regulation gave to the priests the portions of the people's offerings that were reserved from the fire. The verse stands in a chapter that establishes God's order for priestly and sanctuary service, warning against rejecting that order through ambition to take the priests' place and power.
The rule continues the earlier practice of sacrificing firstlings of the flock. The Hebrew word for sprinkling their blood means to scatter or sprinkle, not to dip or immerse.