It is narrated from Sayyiduna Salamah bin Muhabbaq (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) passed by a house, and there was a water skin hanging in the courtyard of that house. The Prophet (peace and blessings be upon him) asked for water, but it was said that it was made from the skin of a dead animal. Upon hearing this, the Prophet (peace and blessings be upon him) said: "Purifying the skin is to tan it." In another narration: "To tan it is to purify it."