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

It is not for the believers to march out altogether. From every division that sets out, a group from each division should remain behind to gain knowledge in the religion, and to warn their people when they return to them, so they may beware.