It is narrated from Abdullah bin Mas'ud (may Allah be pleased with him) that he said: Allah Almighty sends the winds, and they bring water from the sky. Then they pass through the clouds and become filled in such a way (with water) as a pregnant she-camel is filled with milk, then rain is sent down.
Hadith Referenceمسند الإمام الشافعي / كتاب العيدين والأضاحي والاستسقاء / 525
Hadith Takhrijاسناده ضعيف لضعف شيخ الشافعي : اخرجه البيهقى فى المعرفة السنن والآثار (2048) ۔ والطبراني في الكبير : 9/ 223 (8090) ۔ والطبري في التفسير: (20/14)۔