Sunan An-Nasa'i

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