Tafsir of Al Imran 3:166

وما أصابكم يوم التقى الجمعان فبٳذن الله

And what you suffered on the day the two armies met, was by the leave of Allah, for when you ran away from your enemy, who killed many of you and injured many others, all this occurred by Allah's will and decree out of His perfect wisdom.

وليعلم المومنين

in order that He might test the believers. who were patient, firm and were not shaken