Sforno on Exodus 4:1/ספורנו על שמות ד׳:א׳

1והן לא יאמינו לי ולא ישמעו בקולי, once the people will see that Pharaoh will refuse to let them go, they will lose faith in me and will not listen to my promises. 'כי יאמרו לא נראה אליך ה, for they know that when G’d says something it will be so. They will not be able to account for my failure except by claiming that I am an impostor.