Numbers 18:12, 17

“All the best of the oil, all the best of the new wine and the grain, their firstfruits which they offer to the LORD, I have given them to you.

But the firstborn of a cow, 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 burn their fat as an offering made by fire for a sweet aroma to the LORD.