Prayer: Realising the Forehead is on an Invalid Item During Sajdah
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1074
If while performing sajdah one realises that he has placed his forehead on something that invalidates a sajdah, in the event that he becomes aware of this after saying the obligatory dhikr, he can raise his head from sajdah and continue with the prayer. However, if he becomes aware of this before saying the obligatory dhikr, he must slide his forehead onto something on which it is permitted to perform sajdah and then say the obligatory dhikr. If it is not possible for him to slide his forehead, he can say the obligatory dhikr as he is. In both cases, his prayer is valid.