Musnad Ahmad 215:3

'Amr ibn al-Haytham Abu Qatan narrated to us, he said: Hisham narrated to us, from Qatadah, from al-Hasan, from Jun ibn Qatadah, from Salamah ibn al-Muhabbaq, from the Messenger of Allah, peace and blessings of Allah be upon him: "Its tanning is its purification, or its slaughter."