Sforno on Numbers 31:21/ספורנו על במדבר ל״א:כ״א

1זאת חקת התורה, the instructions you have received from Moses concerning your own purification rites, including being sprinkled with water containing ash from the red heifer, is a decree by the Torah. The institution known as פרה אדומה has as its purpose the purification of people afflicted with ritual impurity due to direct or indirect contact with the dead.