מדרש על מלכים א 18:22

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: