Matthew Poole and continuators on Numbers 18:17

17 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 favour unto the LORD.

To wit, in a peculiar manner, and higher degree than other beasts, consecrated to an holy use, even to be sacrificed unto God, and not to be redeemed, nor alienated to any other use. Compare Deut. 15. 19.