Chapter on What is Permissible to Pray in, from Clothing and Place, and What is Not Permissible to Pray in, from That

Al-Hasan ibn Mahbub asked Abu Al-Hasan عليه السلام, peace be upon him, about gypsum (jiss) that had been fired using excrement and the bones of the dead, and then the mosque was plastered with it, whether it is permissible to prostrate upon it. So Imam عليه السلام wrote to him in His عليه السلام own handwriting: “Indeed, water and fire have purified it.”