Numbers 18:12, 17

All the best of the oil, and all the best of the vintage, and of the grain, the first-fruits of them which they give unto Jehovah, to thee have I given them.

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 savor unto Jehovah.