Chapter on the Manner of Prayer, Its Description and What is Obligatory and Recommended

Ahmad ibn Muhammad from Al-Hasan ibn Mahbub, who said: I asked Abu Al-Hasan عليه السلام, about plaster (Gypsum) that had been burned with dung and the bones of the dead, and with which the mosque was plastered, is it permissible to prostrate upon it? So Imam عليه السلام wrote to me in his own handwriting: "Indeed, the water and the fire have purified it."