(Qasas) 28 : 34
وَأَخِي هَارُونُ هُوَ أَفْصَحُ مِنِّي لِسَانًا فَأَرْسِلْهُ مَعِيَ رِدْءًا يُصَدِّقُنِي ۖ إِنِّي أَخَافُ أَنْ يُكَذِّبُونِ
And my brother Haroon is more fluent than me with tongue, so You send him with me as a helper for testifying me; indeed I fear that they will reject me".
See explanation 26: 12-15.