﴿ وَأَخِى هَٰرُونُ هُوَ أَفْصَحُ مِنِّى لِسَانًۭا فَأَرْسِلْهُ مَعِىَ رِدْءًۭا يُصَدِّقُنِىٓ ۖ إِنِّىٓ أَخَافُ أَن يُكَذِّبُونِ ﴾

“And my brother Aaron - he is far better in speech than I am. Send him, therefore, as a helper, so that he might [more eloquently] bear witness to my speaking the truth: for I fear indeed that they will give me the lie.””

(My brother Aaron is more eloquent then me in speech) because Moses had a slight speech impediment. (Therefore send him with me as a helper to confirm me) to speak on my behalf and confirm my words. (Lo! I fear that they will give the lie to me) deny the message I bring to them.