Purity: Recommended Distance for Purifying Sole of Foot or Shoe by Earth
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 179
In order to purify the sole of one’s foot or shoe, it is better to walk a distance of fifteen cubits (dhirāʿs)[1] or more, even if the impurity is removed by walking less than fifteen cubits or by rubbing the sole of one’s foot or shoe on earth.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)