Prayer: Praying on a Prohibited Surface
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 870
Performing prayers on something that is unlawful to stand or sit on – such as a place on a mat that has the name of Allah the Exalted written on it – is not correct in the event that it prevents one from establishing an intention to attain proximity to Allah (qaṣd al‑qurbah).
The fourth condition: the ceiling of the place where one performs prayers must not be so low that he cannot stand up straight, and the place must not be so small that there is no room to perform rukūʿ and sujūd.