Muwatta Malik, Book 25, Hadith 17 (USC-MSA)

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 ‏" ‏ ‏.‏

sunnah.com

also referenced under: Book 25, Hadith 1068 (In-Book)