This is the statute of the Torah that the Lord has commanded, saying: Speak to the children of Yisrael, and they shall take to you a red heifer, perfect, in which there is no blemish, and upon which a yoke has not been laid.
Torah Temimah: Menachos 27a — The three species and the seven sprinklings of the para aduma interlock with one another; the reason is that “chukah” is written concerning it.
English: The three species and the seven sprinklings that are in the [red] heifer interlock with one another. What is the reason? “Chukah” is written concerning it.
Gezeiras ha-kasuv / chukah language that makes the elements interdependent.
Because the Torah calls the para aduma a “chukah,” each of its required elements (cedar, hyssop, crimson, and the seven sprinklings) is indispensable; omission of any one invalidates the whole.