וְהָיָ֥ה עַֽל־אַהֲרֹ֖ן לְשָׁרֵ֑ת וְנִשְׁמַ֣ע ק֠וֹלוֹ בְּבֹא֨וֹ אֶל־הַקֹּ֜דֶשׁ לִפְנֵ֧י יְהוָ֛ה וּבְצֵאת֖וֹ וְלֹ֥א יָמֽוּת׃ (ס)
And it shall be upon Aaron to minister; and the sound thereof shall be heard when he goeth in unto the holy place before the LORD, and when he cometh out, that he die not.
Read kabbalistic (mystical) interpretation of Exodus 28:35: and classical mystical sources.