(a) It is narrated from Sayyiduna Salamah bin al-Muhabbiq al-Hudhali (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Tanning a skin is its purification."
This hadith mentions the purity of the skins of lawful animals. Its routes indicate that what is meant by 'dhakah' is purification.
(b) Mu'adh bin Hisham (may Allah have mercy on him) narrates from his father that he (peace and blessings be upon him) asked a woman for water, and she said: I have (nothing but) a waterskin made from the skin of a carcass. He (peace and blessings be upon him) said: "Had you not tanned it?" The woman said: Of course! He (peace and blessings be upon him) said: "Its purification is indeed its tanning."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 70