Abu Dawud Book 27, Hadith Number 4113.
Chapter : Skins of dead animals.
Narated By Salamah ibn al-Muhabbaq : On the expedition of Tabuk the Apostle of Allah (PBUH) came to a household and, seeing a bucket hanging, asked for water. They said: Apostle of Allah, the animal died a natural earth. He replied; Its tanning is its purification.