Numbers 3:10
And you shall appoint Aaron and his sons, and they shall wait on their priest's office: and the stranger that comes near shall be put to death.
Torrey's Topical Textbook—AKJV

Numbers 16:40
To be a memorial to the children of Israel, that no stranger, which is not of the seed of Aaron, come near to offer incense before the LORD; that he be not as Korah, and as his company: as the LORD said to him by the hand of Moses.
Torrey's Topical Textbook—AKJV

Numbers 18:7
Therefore you and your sons with you shall keep your priest's office for everything of the altar, and within the veil; and you shall serve: I have given your priest's office to you as a service of gift: and the stranger that comes near shall be put to death.
Torrey's Topical Textbook—AKJV