﴿ 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.””

A sermon should have high degree of eloquence هُوَ أَفْصَحُ مِنِّي لِسَانًا (He is more fluent in his tongue - 28:34). This verse points out that a high degree of oratory and eloquence is desirable for sermons and preaching, and there is no harm if one takes training in that.