Musnad Ahmad 675:1

Waki’ and Ibn Ja’far narrated to us, they said: Shu’bah narrated to us, from Al-Hakam, from ‘Abd al-Rahman ibn Abi Layla—Ibn Ja’far said: I heard Ibn Abi Layla—from ‘Abdullah ibn ‘Ukaym al-Juhani, who said: "The letter of the Prophet, peace and blessings of Allah be upon him, came to us while we were in the land of Juhaynah, and I was a young boy: 'Do not make use of the hide or the sinew of a dead animal.'"