Numbers 18:12, 17

All the best of the oil and all the best of the wine and of the grain, the firstfruits of what they give to the LORD, I give to you.

But 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.