Rashi on Numbers 11:18/רש"י על במדבר י״א:י״ח

1התקדשו means, prepare yourselves to receive punishment. Similarly it states, (Jeremiah 12:3) “and prepare them (והקדשם) for the day of slaughter” (Sifrei Bamidbar 94; cf. Rashi on Exodus 19:10).