Exodus 4:1

Then Moses said to Yahweh, “Please, Lord, I have never been a man of words, neither recently nor in time past, nor since You have spoken to Your slave; for I am one with a hard mouth and a hard tongue.”

and Aaron spoke all the words which Yahweh had spoken to Moses. He then did the signs in the sight of the people.

But Moses spoke before Yahweh, saying, “Behold, the sons of Israel have not listened to me; how then will Pharaoh listen to me, for I am of uncircumcised lips?”

Then he gave a miraculous sign the same day, saying, “This is the miraculous sign which Yahweh has spoken, ‘Behold, the altar shall be torn apart and the ashes which are on it shall be poured out.’”

If They Will Not Believe These Signs