لَوْ خَرَجُوا فِيكُم مَّا زَادُوكُمْ إِلَّا خَبَالًا وَلَأَوْضَعُوا خِلَالَكُمْ يَبْغُونَكُمُ الْفِتْنَةَ وَفِيكُمْ سَمَّاعُونَ لَهُمْ ۗ وَاللَّهُ عَلِيمٌ بِالظَّالِمِينَ ٤٧

If they had come out (joining you), they would have added nothing to you except mischief and disorder and would have strived hard amongst you (to stir up disaffection and discord). They want to bring about subversion within you, and (even now some of) their spies are there amongst you. And Allah is Well Aware of the wrongdoers. (9:47)