Exodus 28:35
And it shall be upon Aaron to minister: and his sound shall be heard when he goeth in unto the holy place before the Lord, and when he cometh out, that he die not.
Cross-references
Leviticus 16:2
Historical context
Leviticus 16:2 warns Aaron not to enter the Holy Place arbitrarily lest he die, explaining why the bells' sound is needed for safety.
Hebrews 9:12
Contrast
Hebrews 9:12 contrasts Christ's once-for-all entry into the heavenly Holy Place with the high priest's cautious earthly entry requiring bells.
Hebrews 8:2
Typology
Hebrews 8:2 describes Christ serving in the true tabernacle, which the earthly one with bells and warnings prefigured.