وَما كانَ المُؤمِنونَ لِيَنفِروا كافَّةً ۚ فَلَولا نَفَرَ مِن كُلِّ فِرقَةٍ مِنهُم طائِفَةٌ لِيَتَفَقَّهوا فِي الدّينِ وَلِيُنذِروا قَومَهُم إِذا رَجَعوا إِلَيهِم لَعَلَّهُم يَحذَرونَ

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.