Musnad Ahmad 25:663

'Affan narrated to us, Hammad bin Salamah narrated to us, Zayd bin Aslam informed us, from 'Abd al-Rahman bin Wa'lah, who said: I said to Ibn 'Abbas: We raid the people of the Maghrib, and most of their water skins—and Hammad sometimes said: and the generality of their water skins—are [made from] dead animals. He said: I heard the Prophet, peace and blessings of Allah be upon him, say: "Its tanning is its purification."