Maarif-ul-Quran (En) - Al-Qasas : 34
وَ اَخِیْ هٰرُوْنُ هُوَ اَفْصَحُ مِنِّیْ لِسَانًا فَاَرْسِلْهُ مَعِیَ رِدْاً یُّصَدِّقُنِیْۤ١٘ اِنِّیْۤ اَخَافُ اَنْ یُّكَذِّبُوْنِ
And my brother Harun is more fluent with his tongue than I am; so send him with me as a helper who will bear me out. I am afraid, they will belie me."
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.
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