Rashi on Exodus 28:35/רש"י על שמות כ״ח:ל״ה

1ולא ימות THAT HE DIE NOT — From what is included in this negative statement you may infer the positive: if he has these garments on him he will not incur death; consequently if he enters the Sanctuary lacking one of these garments he incurs death at the hands of God (cf. Sanhedrin 83).