Musnad Ahmad 795:8

Muhammad ibn Ja'far narrated to us, Sa'id narrated to us, from Qatadah, from al-Hasan, from Salamah ibn al-Muhabbaq, that the Prophet of Allah, peace and blessings of Allah be upon him, came upon a water skin on the day of Hunayn. He requested water from it, and there was a woman with it. She said, "It is dead (i.e., the skin of a dead animal)." He said, "Ask her, has it not been tanned?" She said, "Yes." So he took from it for his need, and he said, "The purification of the hide is its tanning."