It is narrated from Hazrat Salamah (may Allah be pleased with him) that once the Prophet (peace and blessings be upon him) passed by a house in whose courtyard a waterskin was hanging. 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 is the purification of the skin."