Malik related to me from Zayd ibn Aslam from Ibn Wala al-Misri from Abdullah ibn Abbas that the Messenger of Allah, may Allah bless him and grant him peace, said, "A skin when it is tanned is pure."

وَحَدَّثَنِي مَالِكٌ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنِ ابْنِ وَعْلَةَ الْمِصْرِيِّ، عَنْ عَبْدِ اللَّهِ بْنِ عَبَّاسٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ إِذَا دُبِغَ الإِهَابُ فَقَدْ طَهُرَ ‏"‏ ‏.‏
USC-MSA web (English) reference : Book 25, Hadith 17
Arabic reference : Book 25, Hadith 1068