At Tawbah, 122 Ayah
9:122
۞ وَمَا كَانَ ٱلۡمُؤۡمِنُونَ لِيَنفِرُواْ كَآفَّةٗۚ فَلَوۡلَا نَفَرَ مِن كُلِّ فِرۡقَةٖ مِّنۡهُمۡ طَآئِفَةٞ لِّيَتَفَقَّهُواْ فِي ٱلدِّينِ وَلِيُنذِرُواْ قَوۡمَهُمۡ إِذَا رَجَعُوٓاْ إِلَيۡهِمۡ لَعَلَّهُمۡ يَحۡذَرُونَ ١٢٢
Wa Mā Kāna Al-Mu'uminūna Liyanfirū Kāffatan ۚ Falawlā Nafara Min Kulli Firqatin Minhum Ţā'ifatun Liyatafaqqahū Fī Ad-Dīni Wa Liyundhirū Qawmahum 'Idhā Raja`ū 'Ilayhim La`allahum Yaĥdharūna
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.