Purity: Unlawful and Permissible Materials for Purifying the Back Passage
Practical Laws of Islam as per the teaching of Ayatullah Sistani
Ruling 66
It is unlawful to purify the anus with things that must be respected, such as paper on which the name of Allah the Exalted and the Prophets are written. There is no problem in purifying the anus with a bone or dung.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)