Purity: Remaining Traces of Impurity on Sole After Walking
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 183
After walking, there is no problem if the smell, colour, or small particles of impurity that cannot be seen remain on the sole of one’s foot or shoe. However, the recommended precaution is that one should walk to the extent that this is also removed.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)