Midrash sur Les Nombres 19:7: Bereshit Rabba et sources classiques

3) "he shall wash his clothes and bathe his flesh in water": I might think that it is a decree of the King (that he must immerse, just as the high-priest does); it is, therefore, written "and then he may come to the camp." Just as "and then he may come to the camp" there (in respect to the red heifer [Bamidbar 19:7]) (means that he may immerse) to free himself of tumah; here, too, (it means) to free himself of tumah, (but if he wishes to prolong his state of tumah, he need not immerse.)

This is the ordinance of the Torah - (Psalms 12:6) The sayings of G-d are pure (purify). R. Hanan Ben Pazzi elucidated this verse [of psalms with the parsha of Parah] Parah- which has seven seven sevens; seven cows, seven fires, seven sprinkling, seven washes, seven unclean, seven pure, seven priests. And if someone tells you they are five, tell him: Moses and Aaron are included, as it is said: And the Lord spoke to Moses and Aaron, saying, This is the ordinance of the Torah: