Tanning purifies the inside of the skin as it does the outside, and it may be used for all liquids

Ibn Wa'lah al-Saba'i said: I asked Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both): We live in the Maghrib (the West), and the Magians bring us waterskins containing water and fat? He said: Drink. I said: Is this an opinion of yours? Ibn Abbas (may Allah be pleased with them both) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Tanning it is its purification."

It is narrated from the Mother of the Believers, Sayyidah Sawdah (may Allah be pleased with her), that a sheep of ours died, so we tanned its skin, and we kept making nabidh in it until it became a (worn-out) waterskin.

Ubaydullah mentioned it with his chain from Sayyidah Maymunah (may Allah be pleased with her), and Simak bin Harb narrated it from Ikrimah.

It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that a sheep belonging to the Mother of the Believers, Sayyidah Sawdah bint Zam'ah (may Allah be pleased with her), died. She said: O Messenger of Allah (peace and blessings be upon him), so-and-so's sheep has died. The Messenger of Allah (peace and blessings be upon him) said to her: "Why did you not take its skin?" She said: (Should we take) the skin of a sheep that has died? The Messenger of Allah (peace and blessings be upon him) said: "Allah Almighty has said: 'Say: I do not find, in what has been revealed to me, anything forbidden to one who would eat it, unless it be a carcass, or blood poured forth, or the flesh of swine' [Surah al-An'am: 145]. You do not eat it; you tan it and derive benefit from it." So she sent someone to it, who skinned it, and she tanned it and made a waterskin from it, until it wore out while with her.

Abu Awanah (may Allah have mercy on him) narrated it, and another narration of the same meaning, with his chain.