صحيح مسلم

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 (ﷺ) say: Tanning purifies it (the skin).