It was narrated from Ibn Wa'lah that he asked Ibn 'Abbas:
We are attacking the Maghrib, and they are people who worship idols, and they have waterskins in which they keep milk and water. Ibn 'Abbas said: Tanning is purification. Ibn Wa'lah said: Is this your own opinion, or something that you heard from the Messenger of Allah? He said: No, (I heard it) from the Messneger of Allah.