Numbers 18:17-19

17But 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 savour unto Yhwh.

18And the flesh of them shall be thine, as the wave breast and as the right shoulder are thine.

19All the heave offerings of the holy things, which the children of Israel offer unto Yhwh, have I given thee, and thy sons and thy daughters with thee, by a statute for ever: it is a covenant of salt for ever before Yhwh unto thee and to thy seed with thee.