Surah At-Taubah (The Repentance), Verse 122

وَمَا كَانَ الْمُؤْمِنُوْنَ لِیَنْفِرُوْا كَآفَّةً فَلَوْلَا نَفَرَ مِنْ كُلِّ فِرْقَةٍ مِّنْهُمْ طَآىِٕفَةٌ لِّیَتَفَقَّهُوْا فِی الدِّیْنِ وَلِیُنْذِرُوْا قَوْمَهُمْ اِذَا رَجَعُوْۤا اِلَیْهِمْ لَعَلَّهُمْ یَحْذَرُوْنَ 

Wa maa kaanal mu'minoona liyanfiroo kaaaffah; falaw laa nafara min kulli firqatim minhum taaa'ifatul liyatafaqqahoo fiddeeni wa liyunziroo qawmahum izaa raja'ooo ilaihim la'allahum yahzaroon

“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).”

Quran 9:122 (Yusuf Ali Translation)