Abu Dawud Book 27, Hadith Number 4115.
Chapter : The view that skins of dead animals should not be made use of.
Narated By Abdullah ibn Ukaym : The letter of the Apostle of Allah (PBUH) 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.