Numbers 4:17-20

And Jehovah spake unto Moses and unto Aaron, saying, Cut ye not off the tribe of the families of the Kohathites from among the Levites; but thus do unto them, that they may live, and not die, when they approach unto the most holy things: Aaron and his sons shall go in, and appoint them every one to his service and to his burden; but they shall not go in to see the sanctuary even for a moment, lest they die.