Aya number 122 of surat At-Taubah
بِسْمِ اللَّهِ الرَّحْمَٰنِ الرَّحِيمِ
وَمَا كَانَ الْمُؤْمِنُونَ لِيَنْفِرُوا كَافَّةً فَلَوْلَا نَفَرَ مِنْ كُلِّ فِرْقَةٍ مِنْهُمْ طَائِفَةٌ لِيَتَفَقَّهُوا فِي الدِّينِ وَلِيُنْذِرُوا قَوْمَهُمْ إِذَا رَجَعُوا إِلَيْهِمْ لَعَلَّهُمْ يَحْذَرُونَ [١٢٢]
Translation: English - Sahih International
And it is not for the believers to go forth [to battle] all at once. For there should separate from every division of them a group [remaining] to obtain understanding in the religion and warn their people when they return to them that they might be cautious.
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