Prayer: Forgetting or Not Knowing a Place is Usurped
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 856
If a person does not know or has forgotten that a certain place is usurped and performs prayers there, and after his prayers, he realises or remembers it is usurped, his prayers are valid. However, if someone has usurped a place himself but forgets and performs prayers there, then based on obligatory precaution, his prayers are invalid.