Rashi on Exodus 4:16/רש"י על שמות ד׳:ט״ז

1ודבר הוא לך means HE WILL SPEAK to the people ON THY ACCOUNT (לְךָ). This passage is proof, regarding every usage of לך and לי and לו and לכם and להם that follow after the verb דבר, that they all have the meaning of על “on account of”.

2יהיה לך לפה HE SHALL BE TO THEE INSTEAD OF A MOUTH — he will be to thee as a spokesman because thou art slow of speech (Exodus Rabbah 3:17).

3לאלהים means a superior and chief (cf. Onkelos).