﴿ وَمَآ أَصَٰبَكُمْ يَوْمَ ٱلْتَقَى ٱلْجَمْعَانِ فَبِإِذْنِ ٱللَّهِ وَلِيَعْلَمَ ٱلْمُؤْمِنِينَ ﴾

“and all that befell you on the day when the two hosts met in battle happened by God's leave, so that He might mark out the [true] believers,”

And what afflicted you the day the two hosts encountered at Uhud was by God’s leave by His will and that He might know through knowledge manifested outwardly the true believers.