Surah At-Tawba, Verse 122
۞ وَمَا كَانَ ٱلْمُؤْمِنُونَ لِيَنفِرُوا۟ كَآفَّةًۭ ۚ فَلَوْلَا نَفَرَ مِن كُلِّ فِرْقَةٍۢ مِّنْهُمْ طَآئِفَةٌۭ لِّيَتَفَقَّهُوا۟ فِى ٱلدِّينِ وَلِيُنذِرُوا۟ قَوْمَهُمْ إِذَا رَجَعُوٓا۟ إِلَيْهِمْ لَعَلَّهُمْ يَحْذَرُونَ
Translation — 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.
Source
Kur'an-ı Kerim, Tevbe Suresi 9:122
https://kuran.diyanet.gov.tr/mushaf/kuran-meal-2/9-suresi-122-ayet