Sforno on Numbers 3:10/ספורנו על במדבר ג׳:י׳

1ושמרו את כהנתם, concerning all matters involving the altar, the dividing curtain and what is beyond it, to ensure that the Levites not enter any of these domains.

2והזר הקרב, any unauthorised person entering these domains will be legally executed (if he did it deliberately after due warning).