Vayikra 16:2

יבא‎ ויאמר ה׳ אל משה דבר אל אהרן אחיך ואל AND THE LORD SPOKE UNTO MOSES, SPEAK UNTO AARON THY BROTHER THAT HE COME NOT [AT ALL TIMES INTO THE HOLY PLACE]! THAT HE DIE NOT as his sons have died (Sifra, Acharei Mot, Section 1 4). ולא ימות THAT HE DIE NOT — for if he comes into the Holy of Holies at any time other than Yom-Kippur he will die (Sifra, Acharei Mot, Section 1 4). כי בענן אראה means, for I constantly show Myself there with My pillar of cloud, and because the revelation of My Shechinah takes place there he should take care not to make it his habit to come there. This is the literal meaning of the verse. The Halachic explanation is: He shall not come into the Holy of Holies except with (i.e. on the occasion when he is going to raise) a cloud of incense on the Day of Atonement (Yoma 53a).

וַיֹּ֨אמֶר יְהֹוָ֜ה אֶל־מֹשֶׁ֗ה דַּבֵּר֮ אֶל־אַהֲרֹ֣ן אָחִ֒יךָ֒ וְאַל־יָבֹ֤א בְכׇל־עֵת֙ אֶל־הַקֹּ֔דֶשׁ מִבֵּ֖ית לַפָּרֹ֑כֶת אֶל־פְּנֵ֨י הַכַּפֹּ֜רֶת אֲשֶׁ֤ר עַל־הָאָרֹן֙ וְלֹ֣א יָמ֔וּת כִּ֚י בֶּֽעָנָ֔ן אֵרָאֶ֖ה עַל־הַכַּפֹּֽרֶת׃

and the Lord said to Moshe, Speak to Aharon thy brother, that he come not at all times into the holy place within the veil before the covering, which is upon the ark; that he die not: for I appear in the cloud upon the ark cover.