Musnad Ahmad 675:6

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from al-Hakam, who said: I heard Ibn Abi Layla narrating from ‘Abdullah ibn ‘Ukaym that he said: The letter of the Messenger of Allah (may Allah bless him and grant him peace) was read to us in the land of Juhaynah while I was a young boy: "Do not make use of the dead animal for its hide, nor its sinew."