Ramban on Exodus 4:5/רמב"ן על שמות ד׳:ה׳

1THAT THEY MAY BELIEVE THAT THE ETERNAL … HATH APPEARED TO THEE. The interpretation of this verse is that “they may believe when you do the sign before them.” Scripture, however, speaks briefly about this for it is self-understood that G-d showed Moses wonders with the intent that he perform them before the people in order that they believe him.