"And We rained upon them a rain," meaning: the stones with which those who were outside of the city on their errands and travels were struck.
﴿وأمطرنا عَلَيْهِم مَطَرا﴾ يَعْنِي: الْحِجَارَةَ الَّتِي رُمِيَ بِهَا مَنْ كَانَ خَارِجًا مِنَ الْمَدِينَةِ فِي حوائجهم وأسفارهم.