Sura al-Tawbah 9:122

وَمَا كَانَ ٱلْمُؤْمِنُونَ لِيَنفِرُوا۟ كَآفَّةً فَلَوْلَا نَفَرَ مِن كُلِّ فِرْقَةٍ مِّنْهُمْ طَآئِفَةٌ لِّيَتَفَقَّهُوا۟ فِى ٱلدِّينِ وَلِيُنذِرُوا۟ قَوْمَهُمْ إِذَا رَجَعُوٓا۟ إِلَيْهِمْ لَعَلَّهُمْ يَحْذَرُونَ

wa-mā kāna l-muʾminūna li-yanfirū kāffatan fa-lawlā nafara min kulli firqatin minhum ṭāʾifatun li-yatafaqqahū fī l-dīni wa-li-yundhirū qawmahum idhā rajaʿū ilayhim la-ʿallahum yaḥdharūna

Nor should the Believers all go forth together: if a contingent from every expedition remained behind, they could devote themselves to studies in religion, and admonish the people when they return to them,- that thus they (may learn) to guard themselves (against evil).