And a pure man shall gather the ashes of the heifer and place them outside the camp in a pure place; and it shall be for the congregation of the children of Yisrael for a keeping, for water of sprinkling; it is a chatas.
Torah Temimah: Chullin 11a — The Merciful One called it a “chatas”; therefore a treifah is invalid.
English: The Merciful One called it a “chatas”; therefore a treifah is invalid.
Analogy from the designation “chatas” to the disqualification of treifah.
Because the para is called a “chatas,” a treifah (an animal with a fatal defect) is disqualified for use as the red heifer.