Surah At-Tawbah, Verse 122

۞ وَمَا كَانَ ٱلۡمُؤۡمِنُونَ لِيَنفِرُواْ كَآفَّةٗۚ فَلَوۡلَا نَفَرَ مِن كُلِّ فِرۡقَةٖ مِّنۡهُمۡ طَآئِفَةٞ لِّيَتَفَقَّهُواْ فِي ٱلدِّينِ وَلِيُنذِرُواْ قَوۡمَهُمۡ إِذَا رَجَعُوٓاْ إِلَيۡهِمۡ لَعَلَّهُمۡ يَحۡذَرُونَ١٢٢

wamā kāna l-mu'minūna liyanfirū kāffatan falawlā nafara min kulli fir'qatin min'hum ṭāifatun liyatafaqqahū fī l-dīni waliyundhirū qawmahum idhā rajaʿū ilayhim laʿallahum yaḥdharūna

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 [i.e., advise] their people when they return to them that they might be cautious.