What Can and Cannot Be Used for Prostration
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 41
وَ سَأَلَ اَلْحَسَنُ بْنُ مَحْبُوبٍ أَبَا اَلْحَسَنِ عَلَيْهِ اَلسَّلاَمُ: عَنِ اَلْجِصِّ يُوقَدُ عَلَيْهِ بِالْعَذِرَةِ وَ عِظَامِ اَلْمَوْتَى ثُمَّ يُجَصَّصُ بِهِ اَلْمَسْجِدُ أَ يُسْجَدُ عَلَيْهِ فَكَتَبَ عَلَيْهِ اَلسَّلاَمُ إِلَيْهِ بِخَطِّهِ «أَنَّ اَلنَّارَ وَ اَلْمَاءَ قَدْ طَهَّرَاهُ.
Al-Hasan ibn Mahbub asked Imam Abu al-Hasan عليه السلام about gypsum (plaster) that was fired using dung and the bones of the dead, and whether it would be permissible to prostrate on it if a mosque was plastered with it. The Imam wrote back in his own handwriting: "Fire and water have purified it”.
- Ṣaḥīḥ al-Faqīh