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It was narrated that Abu `Uthman an-Nahdi said: l was sitting beneath the minbar of `Umar (رضي الله عنه) when he was addressing the people, and he said in his khutbah:I heard the Messenger of Allah (ﷺ) say: “What I fear the most for this ummah is every knowledgeable hypocrite with a smooth tongue.”
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا يَزِيدُ، أَنْبَأَنَا دَيْلَمُ بْنُ غَزْوَانَ الْعَبْدِيُّ، حَدَّثَنَا مَيْمُونٌ الْكُرْدِيُّ، عَنْ أَبِي عُثْمَانَ النَّهْدِيِّ، قَالَ إِنِّي لَجَالِسٌ تَحْتَ مِنْبَرِ عُمَرَ رَضِيَ اللَّهُ عَنْهُ وَهُوَ يَخْطُبُ النَّاسَ فَقَالَ فِي خُطْبَتِهِ سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ إِنَّ أَخْوَفَ مَا أَخَافُ عَلَى هَذِهِ الْأُمَّةِ كُلُّ مُنَافِقٍ عَلِيمِ اللِّسَانِ.
sunnah.com