And Moses said unto the LORD, O my LORD, I am not eloquent, neither heretofore, nor since thou hast spoken
unto thy servant: but I am slow of speech, and of a slow tongue.
And the anger of the LORD was kindled against Moses, and he said, Is not Aaron the Levite thy brother? I know that he can speak well.
... And thou shalt speak unto him, and
put words in his mouth: and I will be with thy mouth, and with his mouth, and will teach you what ye shall do. And he shall be thy spokesman
unto the people: and he shall be, even he shall be to thee instead of a mouth, and thou shalt
be to him instead of God. Exodus 4:14-16
And Moses spake before the LORD, saying, Behold, the children of Israel have not hearkened unto me;
how then shall Pharaoh hear me, who am of uncircumcised lips?
And Moses said before the LORD, Behold, I am of uncircumcised lips, and how shall
Pharaoh hearken unto me? Exodus 6:30