Purity: Purification by Collected Rainwater During Rainfall

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 40

Whenever rainwater collects in a place – even if its quantity is less than kurr in the event that an impure object is washed in it while it is raining and the water does not acquire the smell, colour, or taste of the impurity, the impure object becomes pure.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)