At Tawbah
Ayah-129, madinah
9:122
وَمَا كَانَ ٱلۡمُؤۡمِنُونَ لِيَنفِرُواۡ كَآفَّةًۚ فَلَوۡلَا نَفَرَ مِن كُلِّ فِرۡقَةٍ مِّنۡهُمۡ طَآئِفَةٌ لِّيَتَفَقَّهُواۡ فِى ٱلدِّينِ وَلِيُنذِرُواۡ قَوۡمَهُمۡ إِذَا رَجَعُوٓاۡ إِلَيۡهِمۡ لَعَلَّهُمۡ يَحۡذَرُونَ١٢٢
Saheeh 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.