It is narrated from Hazrat Salamah (may Allah be pleased with him) that once, on the occasion of the Battle of Hunayn, the Prophet (peace and blessings be upon him) passed by a house where a water skin was hanging in the courtyard. The Prophet (peace and blessings be upon him) asked those people for water to drink, so they said that it was made from the skin of a dead animal. The Prophet (peace and blessings be upon him) said, "Tanning purifies the skin."