The letter of the Messenger of Allah (ﷺ) was read out to us in the territory of Juhaynah when I was a young boy: Do not make use of the skin or sinew of an animal which died a natural death.
Sunan Abi Dawud Book 34, Hadith 108
← Previous Hadith I had some sheep at Uhud, and they began to die. I then entered upon Maymunah, wife of the Prophet (ﷺ), and mentioned it to her. Maymunah said to me: If you took their skins and made use of them, (that would be better for you). She asked: Is that lawful? She replied, Yes. Some people of the Quraysh passed by the Messenger of Allah (ﷺ) dragging a sheep of theirs as big as an ass. The Messenger of Allah (ﷺ) said to them: Would that you took its skin. They said: It died a natural death. The Messenger of Allah (ﷺ) said: Water and leaves of the mimosa flava purify it.

Sunan Abi Dawud Book 34, Hadith 107 Next Hadith → They entered and I sat at the door. Then they came out and told me that Abdullah ibn Ukaym had informed them that the Messenger of Allah (ﷺ) had written to Juhaynah one month before his death: Do not make use of the skin or sinew of an animal which died a natural death.

Abu Dawud said: Al-Nadr b. Shumail said: The skin is called ihab when it is not tanned and when it is tanned, it not called ihab but na,es shann and qirbah (tanned skin or leather).

Sunan Abi Dawud Book 34, Hadith 109