Rashi on Exodus 18:4/רש"י על שמות י״ח:ד׳

1ויצלני מחרב פרעה AND HE DELIVERED ME FROM THE SWORD OF PHARAOH — When Dathan and Abiram made the disclosure regarding the matter of the Egyptian whom Moses had killed and Pharaoh wished to slay Moses, his neck became as a column of marble so that the sword was powerless against him (Shemot Rabbah 1:31; cf. Rashi on Exodus 2:15).