Prayer: Ignorance of the Prayer Ruling in a Usurped Place
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 857
If someone knows that a certain place is usurped and that using it is unlawful, but he does not know that there is a problem in performing prayers in a usurped place and performs prayers there, then based on obligatory precaution, his prayers are invalid.