Parashas Yisro · Passuk 19:22
Chabad.org — English translation + Rashi
Torah Temimah points to see Torah Temimah note (verified).
English: Rabbi Yochanan ben Beroka says: This refers to the separation of the firstborn; Rabbi says: This refers to the separation of Nadav and Avihu.
In context: See the cited source for the full section.
Derash on the verse.
Even those who normally approach the divine service must maintain separation at the moment of revelation.