Purity: Tayammum in a Usurped Location
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 684:
Tayammum performed in a usurped area is not invalid; therefore, if a person strikes his hands on soil belonging to him, and then enters the property of another person without his consent and wipes his hands on his forehead, his tayammum is valid even though he will have sinned.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)