مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2423
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Zaid bin Aslam said: 'Abdur-Rahman bin Wa'lah said: I said to Ibn 'Abbas (رضي الله عنه): We go out on campaign and people bring animal skins and waterskins to us. He said: I do not know what to say to you except that I heard the Messenger of Allah (ﷺ) say: "Any animal skin that has been tanned has become tahir (pure)."
Musnad Ahmad 2435Its isnad is saheeh, Muslim (366)]