Purity: Ruling on Carpet and Ground Purified by Rainwater
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 41
If rain falls on a carpet that is pure and which is spread on ground that is impure, and if while it is raining the water soaks through the carpet and comes into contact with the ground, the carpet does not become impure and the ground becomes pure.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)