John Gill Commentary Exodus 4:12

"Now therefore go, and I will be with thy mouth, and teach thee what thou shalt speak." — Exodus 4:12 (ASV)

Now therefore go, and I will be with your mouth
And put words into it, and cause it to speak readily and powerfully; and so it appears that he was mighty in words, as well as in deeds, (Acts 7:22) : and teach you what you shall say ;
to Pharaoh, to the Israelites, and to Aaron, that was to speak for him, as is hereafter observed.