﴿ فَلَمْ تَقْتُلُوهُمْ وَلَٰكِنَّ ٱللَّهَ قَتَلَهُمْ ۚ وَمَا رَمَيْتَ إِذْ رَمَيْتَ وَلَٰكِنَّ ٱللَّهَ رَمَىٰ ۚ وَلِيُبْلِىَ ٱلْمُؤْمِنِينَ مِنْهُ بَلَآءً حَسَنًا ۚ إِنَّ ٱللَّهَ سَمِيعٌ عَلِيمٌۭ ﴾

“And yet, [O believers,] it was not you who slew the enemy, but it was God who slew them; and it was not thou who cast [terror into them, O Prophet], when thou didst cast it, but it was God who cast it: and [He did all this] in order that He might test the believers by a goodly test of His Own or daining. Verily, God is all-hearing, all-knowing!”

Please check ayah 8:18 for complete tafsir.