مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2510
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that 'Abdur-Rahman bin Wa'lah said: I said to Ibn 'Abbas (رضي الله عنه): We are campaigning against the people of the Maghrib and most of their waterskins are made from animals that were not slaughtered properly. He said: I heard the Prophet (ﷺ) say: "Its tanning is its purification."
Musnad Ahmad 2522Its isnad is saheeh, Muslim (366)]