Purity: Purification of the Back Passage with Stone, Earth, or Cloth
Practical Laws of Islam as per the teaching of Ayatullah Sistani
Ruling 64
The anus can be purified with stone, a clod of earth, cloth, or a similar thing if they are dry and pure; and there is no problem if they have a little moisture that does not wet the outlet.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)