Rashi on Exodus 2:23/רש"י על שמות ב׳:כ״ג

1[ויהי בימים הרבים ההם AND IT CAME TO PASS DURING THOSE MANY DAYS during which Moses was sojourning in Midian, וימת מלך מצרים THAT THE KING OF EGYPT DIED, and the Israelites felt the need of help; and therefore ומשה היה רעה “And Moses fed the flock” and help came through him. For this reason these chapters are placed in juxtaposition].

2וימת מלך מצרים THE KING OF EGYPT DIED — he became stricken with leprosy (and therefore may be spoken of as dead; cf. Numbers 12:12), and he used to slaughter Israelitish children and bathe in their blood as a cure for his disease (cf. Targum Jonathan and Exodus Rabbah 1:34).