Musnad Ahmad 25:679

Bahz narrated to us, Hammad ibn Salamah narrated to us, Zayd ibn Aslam narrated to us, from Abd al-Rahman ibn Wa'lah, who said: I asked Ibn Abbas, I said: "We raid this Maghrib (the West), and most of their water skins are the hides of dead animals?" He said: He replied: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: 'Its tanning is its purification.'"