﴿ قَالَ سَنَشُدُّ عَضُدَكَ بِأَخِيكَ وَنَجْعَلُ لَكُمَا سُلْطَٰنًۭا فَلَا يَصِلُونَ إِلَيْكُمَا ۚ بِـَٔايَٰتِنَآ أَنتُمَا وَمَنِ ٱتَّبَعَكُمَا ٱلْغَٰلِبُونَ ﴾

“Said He: “We shall strengthen thine arm through thy brother, and endow both of you with power, so that they will not be able to touch you: by virtue of Our messages shall you two, and all who follow you, prevail!””

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.