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).
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. Every act in the
battle is ascribed to Allah, a it was in His cause and it
was not undertaken except by His command. (R)
Yusuf Ali, Abdullah. Translation and Commentary of the Quran. Microsoft Word – 008 Anfal.doc (quran4u.com)