It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Any skin that is tanned becomes pure."
(b) Imam al-Bayhaqi (may Allah have mercy on him) said: All the narrators agree on the mention of tanning in this hadith.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 49