And with this same chain of transmission from Abu Abdillah عليه السلام, who said:
I asked Imam عليه السلام about the extent of mud upon which it is not permissible to prostrate, what is it? Imam عليه السلام said: "When forehead sinks into it and does not remain stable on the ground."
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 2, p. 336