Numbers 18:12, 17

All the best of the fresh [olive] oil, and all the best of the new 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 a cow or of a sheep or of a goat you shall not redeem; they are holy [and belong to the LORD]. You shall sprinkle their blood on the altar and shall offer up their fat in smoke as an offering by fire, for a sweet and soothing aroma to the LORD.