Aal-i-Imraan 3:166

وَمَآ أَصَٰبَكُمْ يَوْمَ ٱلْتَقَى ٱلْجَمْعَانِ فَبِإِذْنِ ٱللَّهِ وَلِيَعْلَمَ ٱلْمُؤْمِنِينَ ١٦٦

And what struck you on the day the two armies met was by permission of Allah that He might make evident the [true] believers.