The Quran

Preface

إِذ يُغَشّيكُمُ النُّعاسَ أَمَنَةً مِنهُ وَيُنَزِّلُ عَلَيكُم مِنَ السَّماءِ ماءً لِيُطَهِّرَكُم بِهِ وَيُذهِبَ عَنكُم رِجزَ الشَّيطانِ وَلِيَربِطَ عَلىٰ قُلوبِكُم وَيُثَبِّتَ بِهِ الأَقدامَ

Yusuf Ali

Remember He covered you with a sort of drowsiness,¹ to give you calm as from Himself, and he caused rain to descend on you² from heaven, to clean you therewith, to remove from you the stain of Satan,³ to strengthen your hearts, and to plant your feet firmly therewith.

Cf. 3:154 for Uḥud. Calm (presence of mind) is essential in battle and in all posts of danger. If the mind is too much in a state of excitement, it cannot carry out a well-considered or well-concerned plan. This spirit of calm confidence on the part of the Muslims won against the blustering violence of the Quraish.
The rain was welcome for many reasons. (1) Water was scarce both for drinking and ablutions; (2) the Muslim band, without baggage or equipment or comforts, found that their thirst aggravated their fatigue; (3) the sand was loose, and the rain consolidated it and enabled them “to plant their feet firmly.”
“Stain of Satan”: both literally and figuratively. Dirt is physically a symbol of evil, and the Muslims were particular about ablutions before prayer. But the rain also refreshed their spirits and removed any lurking doubts in their minds (suggestions of the Evil One) that victory might be impossible in such adverse circumstances.