Exodus 6:12

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? He argues that if the less difficult was impossible for him, the harder task would certainly be far above his ability. He assigns the reason for his failure to his uncircumcised, his unregenerate lips, which made it impossible for him to transmit the words of the Lord to Aaron with the proper ease and fluency and in all their stainless purity.