I heard the Messenger of Allah ﷺ say, "When a hide is tanned, it becomes pure."
سمعت رسول الله ﷺ يقول: إذا دبغ الإهاب فقد طهر.
Muslim narrated it in the Book of Menstruation (366:105) from Yahya bin Yahya, who said, Sulayman bin Bilal informed us, from Zayd bin Aslam, that Abdul-Rahman bin Waalah informed him, from Abdullah bin Abbas, and he related it. Malik also narrated it in the Book of Hunting (17) from Zayd bin Aslam with the same wording.