Sahih Muslim – Hadith No : 2120

Anas (b. Malik) reported: It rained upon us as we were with the Messenger of Allah (may peace be upon him). The Messenger of Allah (way peace be upon him) removed his cloth (from a part of his body) till the rain fell on it. We said: Messenger of Allah, why did you do this? He said: It is because it (the rainfall) has just come from the Exalted Lord.

Hadith In Arabic Text :

وحدثنا يحيى بن يحيى، أخبرنا جعفر بن سليمان، عن ثابت البناني، عن أنس، قال قال أنس أصابنا ونحن مع رسول الله صلى الله عليه وسلم مطر قال فحسر رسول

Chapter : The Book of Prayer (Rain) – كتاب صلاة الاستسقاء‏