Surah At-Tawbah
۞ وَمَا كَانَ ٱلْمُؤْمِنُونَ لِيَنفِرُوا۟ كَآفَّةً ۚ فَلَوْلَا نَفَرَ مِن كُلِّ فِرْقَةٍ مِّنْهُمْ طَآئِفَةٌ لِّيَتَفَقَّهُوا۟ فِى ٱلدِّينِ وَلِيُنذِرُوا۟ قَوْمَهُمْ إِذَا رَجَعُوٓا۟ إِلَيْهِمْ لَعَلَّهُمْ يَحْذَرُونَ
Wa mā kānal-mu'minūna liyanfirū kāffah(tan), falau lā nafara min kulli firqatim minhum ṭā'ifatul liyatafaqqahū fid-dīni wa liyunżirū qaumahum iżā raja‘ū ilaihim la‘allahum yaḥżarūn(a).
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.