Rashi on Numbers 16:12/רש"י על במדבר ט״ז:י״ב

1וישלח משה וגו׳ AND MOSES SENT [TO CALL DATHAN AND ABIRAM] — From here we may learn that one should not persist in strife (Midrash Tanchuma, Korach 10), for, you see, Moses sought them out in order to conciliate them by peaceful words (Sanhedrin 110a).

2לא נעלה WE WILL NOT GO UP — Their mouth tripped them up (i.e. unwittingly they made mention of their fate) — that they would have only a “descent” (their words are taken to mean: we shall not be going up; we shall go down into the depths of the earth) (Midrash Tanchuma, Korach 6).