I heard the Messenger of Allah ﷺ say, "Any hide that has been tanned has become pure."
حدثنا أبو بكر حدثنا سفيان بن عيينة عن زيد بن أسلم عن عبد الرحمن بن وعلة
سمعت رسول الله ﷺ يقول أيما إهاب دبغ فقد طهر
Muslim narrated it in the chapter on menstruation, section titled "The Purification of the Skins of Dead Animals by Tanning," hadith no. 105/366, from Abu Bakr bin Abu Shayba, through this same chain.