Exodus 6:30

Then Moses answered and said, “What if they will not believe me and will not listen to my voice? For they may say, ‘Yahweh has not appeared to you.’”

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 I said, “Woe is me, for I am ruined! For I am a man of unclean lips, And I live among a people of unclean lips; For my eyes have seen the King, Yahweh of hosts.”