The correct opinion [in this matter] is what is narrated in some wordings of the ḥadīth: ‘Tanning [the hide of an animal] is through its lawful slaughtering.’ The word used is ‘slaughtering’ which only purifies the animal that is permissible for eating. For example, if you were to slaughter a donkey [impermissible for consuming], and you proclaimed the name of Allāh on it, and allowed the blood to run, this would not be termed lawful slaughter. Based on this, we say:

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The hide of any animal which is ḥarām for us to consume, even if that animal is considered pure while alive, is not purified by tanning.

The reasoning for this: Any animal that is considered pure while alive is given such a ruling based on the difficulty in avoiding that animal as evidenced by his saying (may the peace and blessings of Allāh be upon him) [concerning domestic cats]: ‘It is only from among the animals that are constantly roaming around you.’ This reasoning becomes obsolete when the animal dies in which case its ruling returns to its origin which is impurity. Thus, its hide is not purified by the tanning process.

Based on this, the correct opinion is that any animal fit for consumption that dies will have its hide purified through tanning. This is one of the two opinions held by Shaykh al-Islām Ibn Taymiyyah (may Allāh have mercy on him).

Source: Al-Sharḥ al-Mumtiʿ 1:92
Translated by: Riyāḍ al-Kanadī