Purity: Tayammum with Mud and Damp Earth

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 680:

Performing tayammum with a mud wall is valid. The recommended precaution is that if there is dry earth or soil, one should not perform tayammum with damp earth or soil.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)