Malik related to me from Zayd ibn Aslam from Ibn Wala al-Misrifrom Abdullah ibn Abbas that the Messenger of Allah, may Allah blesshim and grant him peace, said, "A skin when it is tanned is pure." (ES)
USC-MSA web (English) reference: Book 25, Hadith 17 (ES)
Arabic reference (ES): Book 25, Hadith 1068 (ES)