Rashi on Exodus 8:12/רש"י על שמות ח׳:י״ב

1אמר אל אהרן SAY UNTO AARON — The dust did not deserve to be smitten by Moses because it had protected him when he slew the Egyptian, for “he hid him in the sand”; and it was therefore smitten by Aaron (Tanchuma; Exodus Rabbah 10:7).