Translation:

17. It is not ye who slew them; it was Allah: when thou threwest (a handful of dust) it was not thy act but Allah's: in order that He might test the believers by a gracious trial from Himself: for Allah is He who heareth and knoweth (all things).

Notes (Tafseer)

1191. When the battle began, the Holy Prophet prayed, and threw a handful of dust or sand at the enemy, which, as described in traditions, struck the eyes of the enemy. This had a great psychological effect,

1192. Numerically the odds against the Muslims were three to one. In other ways they were at a disadvantage: of arms and equipment they had but little, while the enemy were well-found: they were inexperienced, while the Quraish had brought their foremost warriors. In all this there was a test, but the test was accompanied by gracious favours of countless value: their Commander was one in whom they had perfect faith, and for whom they were ready to lay down their lives; the rain refreshed them; their spirit was unshaken; and they were fighting in Allah's cause. Thus the trial or test became itself a blessing.