Purity: Ruling on Impure Palms, Knees, and Objects That Do Not Become Pure by Movement

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 182

Based on obligatory precaution, if the palms or knees of someone who moves around on his hands and knees become impure, they do not become pure as a result of him moving around on them. The same applies to the bottom of a walking stick, the bottom of an artificial leg, the shoe of a quadruped animal, the wheel of a motorcycle or car, and similar things.

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