Rashi on Numbers 26:11/רש"י על במדבר כ״ו:י״א

1ובני קרח לא מתו BUT THE SONS OF KORAH DID NOT DIE — They were in the plot originally, but at the moment when the rebellion broke out they had thoughts of repentance in their hearts; therefore a high spot was fenced round for them in Gehinnom and they stayed there (Sanhedrin 110a).