Musnad Ahmad 25:1019

Yahya ibn Adam narrated to us, Mis'ar narrated to us, from 'Amr ibn Murrah, from Salim ibn Abi al-Ja'd, from his brother, from Ibn Abbas, who said: The Prophet, peace and blessings of Allah be upon him, wanted to perform ablution from a water skin, and it was said to him: "It is a dead animal (i.e., the skin of an animal that died without slaughter)." He said: "Its tanning removes its impurity, or its filth, or its uncleanness."