Leviticus 16:2 NIrV

[2] The LORD said to Moses, “Speak to your brother Aaron. Tell him not to come into the Most Holy Room just anytime he wants to. Tell him not to come behind the curtain in front of the cover of the ark. The cover is the place where sin is paid for. If he comes behind the curtain, he will die. That is because I appear in the cloud over the cover.