Leviticus 16:1

And the Lord spake unto Moses after the death of the two sons of Aaron, when they offered before the Lord, and died;

Cross-reference

Leviticus 10:1 recounts the death of Aaron's sons Nadab and Abihu, which is the event referenced here as the occasion for the Day of Atonement instructions.

Leviticus 10:2 recounts the death of Nadab and Abihu by fire from the Lord — this event is the immediate background for the warning in Leviticus 16:1.

Exodus 30:20 similarly warns that priests must wash or die when approaching the altar — both stress proper preparation to avoid death in God's presence.