Ezra 2:62
These sought their registration among those enrolled in the genealogies, but they were not found there, and so they were excluded from the priesthood as unclean.
Numbers 3:10
And you shall appoint Aaron and his sons, and they shall guard their priesthood. But if any outsider comes near, he shall be put to death."
Leviticus 21:21-23
No man of the offspring of Aaron the priest who has a blemish shall come near to offer the LORD's food offerings; since he has a blemish, he shall not come near to offer the bread of his God.
Numbers 16:39-40
So Eleazar the priest took the bronze censers, which those who were burned had offered, and they were hammered out as a covering for the altar,
Numbers 18:7
And you and your sons with you shall guard your priesthood for all that concerns the altar and that is within the veil; and you shall serve. I give your priesthood as a gift, and any outsider who comes near shall be put to death."
Ezekiel 44:10-14
But the Levites who went far from me, going astray from me after their idols when Israel went astray, shall bear their punishment.