Rain in the Night

The following day, in the morning, they had rain, and this was not the season for it. Ibn Hishām says the valley was soft ground, and the water from the sky dampened the earth for the Messenger of Allah (peace be upon him) and his force, but did not impede their progress, while such rain fell upon Quraysh that they could not move ahead. This is rain that showered on both the Muslims and the disbelievers, but for the Muslims it dampened the earth and made it firm, while with the people of Quraysh it became muddy and sticky and impeded their progress. So the same rain had a different effect on both sides — it is a muʿjizah, a miracle, from Allah, the Almighty and Majestic.

Also, some of the Muslims woke up in a state of impurity, having had wet dreams, and a Muslim does not really feel comfortable in a state of impurity — one wants to wash as soon as possible. So that they would spare them that feeling of going into battle while in a state of najāsah, Allah, the Almighty and Majestic, sent down this water to cleanse them. This is a spiritual cleansing for them, in addition to its being water that would strengthen and make the earth firm for them to march over.

Remember when He overwhelmed you with drowsiness, a security from Him, and sent down upon you from the sky rain, by which to purify you and remove from you the evil suggestions of Shayṭān, and to make steadfast your hearts and plant firmly thereby your feet. This rain served all of these purposes.