Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.015.
Section : Purifying the skin of dead animals.
Tanning purifies the skin and the fur of a dead animal. This is based on the hadith of Ibn ‘Abbas, in which the Prophet said, “If the animal’s skin is tanned, it is purified.” (Related by al-Bukhari and Muslim.)