Sahih Muslim Book 3, Hadith Number 713.

Chapter : Purification of the skins of the dead animals by tanning them.

Ibn Wa’la al-Saba’i reported: I asked ‘Abdullah b. ‘Abbas saying: We are the inhabitants of the western regions. The Magians come to us with skins full of water and fat. He said: Drink. I said to him: Is it your own opinion? Ibn Abbas said: I heard the Messenger of Allah (may peace be upon him) say: Tanning purifies it (the skin).