Al-Husayn ibn Sa‘id from Hammad ibn Isa from Hariz from Muhammad ibn Muslim who said:
I asked Him عليه السلام about the skin of a dead animal, can it be worn in prayer if it has been tanned? Imam عليه السلام said: "No, even if it is tanned seventy times."
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 2, p. 215