The Quran

فَلَم تَقتُلوهُم وَلٰكِنَّ اللَّهَ قَتَلَهُم ۚ وَما رَمَيتَ إِذ رَمَيتَ وَلٰكِنَّ اللَّهَ رَمىٰ ۚ وَلِيُبلِيَ المُؤمِنينَ مِنهُ بَلاءً حَسَنًا ۚ إِنَّ اللَّهَ سَميعٌ عَليمٌ

Muhammad Asad

And yet, [O believers,] it was not you who slew the enemy,¹ 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:² and [He did all this] in order that He might test the believers by a goodly test of His Own or daining.³ Verily, God is all-hearing, all-knowing!

Lit., "you did not slay them - i.e., in the battle of Badr, which ended with a complete victory of the Muslims.
According to several Traditions, the Prophet cast, at the beginning of the battle, a handful of pebbles or dust in the direction of the enemy, thus symbolically indicating their approaching defeat. However, none of these accounts attains to the standard of authenticity described as sahih (i.e., "reliable") by the great exponents of the science of Tradition ('ilm al-hadith), and cannot, therefore, satisfactorily explain the above Qur'anic passage (see Ibn Kathir's commentary on this verse, as well as Manar IX, 620 f.). Since the verb rams (lit., "he cast" or "flung") applies also to the act of "shooting an arrow" or "flinging a spear", it might be explained here as a reference to the Prophet's active participation in the battle. Alternatively, it may denote his "casting terror", i.e., into the hearts of his enemies, by his and his followers' extreme valour. Whichever explanation is adopted, the above verse implies that the victory of the Muslims over the much more numerous and much better equipped army of the Quraysh was due to God's grace alone: and, thus, it is a reminder to the faithful, of all times, not to indulge in undue pride in any of their achievements (which is the meaning of the "test" mentioned in the next sentence).
Lit., "from Himself".