Ibn Wa'lah al-Saba'i said: I asked Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both): We live in the Maghrib (the West), and the Magians bring us waterskins containing water and fat? He said: Drink. I said: Is this an opinion of yours? Ibn Abbas (may Allah be pleased with them both) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Tanning it is its purification."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 53