Purity: Tayammum While Imprisoned in a Usurped Place
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 686:
If a person is imprisoned in a usurped place and both the water and the soil of that place are usurped, he must perform prayers with tayammum; however, when he performs tayammum, he must not strike his hands on the ground; rather, he must suffice with placing them on the ground.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)