Rashi on Exodus 4:3/רש"י על שמות ד׳:ג׳

1ויהי לנחש AND IT BECAME A SERPENT — This was an indication to him, that he had slandered the Israelites by saying (v. 1) “But, behold they will not believe me etc.”, and that he had made the serpent’s occupation (slander) his own (cf. Genesis 3:5) (Exodus Rabbah 3:12).