Musnad Ahmad 1153:1196

Hajjaj narrated to us, Sharik and Husayn informed us, Sharik narrated to us, from al-A’mash Sulayman, from ‘Umarah ibn ‘Umayr, from al-Aswad, from ‘Aishah, from the Prophet, may Allah bless him and grant him peace, she said: The Prophet, may Allah bless him and grant him peace, was asked about the skins of a dead animal. He said: "Its tanning is its purification."