At-Tawbah 9:122

Wama kana almu/minoonaliyanfiroo kaffatan falawla nafara min kullifirqatin minhum ta-ifatun liyatafaqqahoo fee addeeniwaliyunthiroo qawmahum itha rajaAAoo ilayhimlaAAallahum yahtharoon

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 [i.e., advise] their people when they return to them that they might be cautious.

It is not (necessary) for all the believers to go forth.’; So, why should it not be that a group from every section of them goes forth, so that they may acquire perfect understanding of the Faith, and so that they may warn their people when they return to them, so that they may take due care (of the rules of Sharī‘ah ).