Another matter, “speak to the priests” – that is what is written: “Fear of the Lord is pure, it endures forever” (Psalms 19:10). Rabbi Levi said: Due to the fear that Aaron had before the Holy One blessed be He, he merited, and this portion, which will not move from him, not from his sons, not from the sons of his sons, until the end of all generations, was given to him. Which [portion] is that? The portion of the corpse, as it is stated: “The Lord said to Moses: Speak to the priests, sons of Aaron...”
What the Phrase Speak to the Priests Truly Conceals
Vayikra Rabbah 26:6