Purity: Prohibition of Destroying Graves of Religious Figures
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 631:
It is unlawful to destroy the graves of the descendants of the Infallible Imams (ʿA), martyrs, and scholars, in any way that is considered disrespectful, even if many years have passed since the person was buried and the body has completely decomposed.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)