3:166
وَمَآ أَصَـٰبَكُمْ يَوْمَ ٱلْتَقَى ٱلْجَمْعَانِ فَبِإِذْنِ ٱللَّهِ وَلِيَعْلَمَ ٱلْمُؤْمِنِينَ ١٦٦
Saheeh International
And what struck you on the day the two armies met was by permission of Allah that He might make evident the [true] believers.