Numbers 18:12, 17

All the best of the fresh oil and all the best of the fresh wine and of the grain, the first fruits of those which they give to the LORD, I give them to you.

But the firstborn of an ox 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 offer up their fat in smoke as an offering by fire, for a soothing aroma to the LORD.