Ramban on Shemos 4:15

English: AND I WILL BE WITH THY MOUTH. I.e., “to teach you that which you are to speak to Pharaoh.” Hashem now told Moshe that Aharon will speak on his behalf only to the people, as it is said, And he shall be thy spokesman unto the people , 284 Verse 16. but Moshe himself was to speak to Pharaoh. It is possible that this was out of respect to the king. But in the end, Moshe came back and said, Behold, I am of uncircumcised lips, and how shall Pharaoh hearken unto me? 285 Further, 6:30. Hashem then permitted him that he should not speak even to Pharaoh. This [consent] was a distinction to Moshe, and therefore He said there, See, I have set thee in Hashem’s stead to Pharaoh , and Aharon thy brother shall be thy prophet . 286 Ibid. , 7:1 And the intent of the expression [here in the verse], and I will be with his [Aharon’s] mouth , 287 I.e., why was this promise necessary since Aharon was not defective of speech? is that his words will find favor with everyone that will hear them.