Praying on Usurped Property
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 853
The place where prayers are performed must fulfil seven conditions.
The first condition: it must be permissible to use [i.e. it must not be usurped], based on obligatory precaution.
Ruling 853. If someone performs prayers on usurped property, even if it is a carpet, couch, or something similar, then based on obligatory precaution, his prayers are invalid. However, there is no problem in performing prayers under a usurped roof or in a usurped tent.