Surah Ali 'Imran Verse 166

وَمَا أَصَابَكُمْ يَوْمَ الْتَقَى الْجَمْعَانِ فَبِإِذْنِ اللَّهِ وَلِيَعْلَمَ الْمُؤْمِنِينَ

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