Musnad Ahmad 25:577

'Abd al-Razzaq narrated to us, Sufyan informed us, from Zayd ibn Aslam, who said: 'Abd al-Rahman ibn Wa'lah narrated to me, from Ibn 'Abbas, who said: I said to him: We go on military expeditions, and we are brought skins and water skins. He said: I do not know what to say to you, except that I heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: "Any skin that is tanned has become pure."