The awe of the LORD is pure (Psalm 19:10) — said Rabbi Levi, since Aaron would revere the Name, as it is written And I gave to him awe and he revered Me. (Malachi 2:5) thus a single section of the Torah was given to him, which will not move from his sons or his sons' sons until the end of all generations. And which is this? The portion of the dead, to not be defiled by death. As it written, Say to the priests the children of Aaron... (Leviticus 21:1).
And correct together (Psalm 19:10) — R. Shim'on son of Laqish said, "Correct by means of an a fortiori." But the rabbis said, "Correct by means of a verbal analogy."...ALTERNATIVE translation: The fear of God is pure. Rabbi Levi said, "Since Aaron feared the name of God, as it is said, 'And I gave them to him as a possession, and they feared me' (Malachi 2:5), therefore a single section of the Torah was given to him that does not move, neither from his sons nor from his grandsons, until the end of all generations."
And which section is this? It is the section of the red heifer, so that he should not become impure through contact with the dead, as it is said, "Speak to the priests, the sons of Aaron" (Leviticus 21:1). "They shall be righteous together" (Isaiah 45:24). Rabbi Shimon ben Lakish said, "They shall be righteous by an a fortiori argument." The Sages said, "They shall be righteous by means of a verbal analogy."