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Surah 8. Al-Anfal
Ayah 11
١١. إِذْ يُغَشِّيكُمُ ٱلنُّعَاسَ أَمَنَةً مِّنْهُ وَيُنَزِّلُ عَلَيْكُم مِّنَ ٱلسَّمَآءِ مَآءً لِّيُطَهِّرَكُم بِهِۦ وَيُذْهِبَ عَنكُمْ رِجْزَ ٱلشَّيْطَـٰنِ وَلِيَرْبِطَ عَلَىٰ قُلُوبِكُمْ وَيُثَبِّتَ بِهِ ٱلْأَقْدَامَ
[11] And remember the time when Allah was giving you peace and security from Himself in the form of drowsiness,8 and was sending down on you water from heaven in order to cleanse you and to remove from you the defilement caused by Satan, and to strengthen your hearts and to steady your footsteps.9
8The Muslims experienced the same thing during the Battle of Uhd. (III: 154). On both these critical occasions, Allah filled the hearts of the Muslims with such peace and security that they were overpowered by drowsiness.
9On the eve of the Battle of Badr, there was a rainfall, which helped the Muslims in three ways. First, they got an opportunity of storing water in reservoirs. Secondly, it made the sandy ground hard in the upper part of the valley where the Muslims were camping and they could move about with steady footsteps. Thirdly, it created difficulties for the army of the disbelievers who were camping in the lower part of the valley for the rain-water gathered there and made the ground marshy and their feet sank into the mud.
"The defilement caused by Satan" was the fear and confusion he had imbued in their minds because of the coming battle. The God-sent rainfall and slumber at that critical occasion helped to remove that fear and confusion.