Musannaf Ibn Abi Shayba 16:1048

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from al-Hakam, from 'Abd al-Rahman ibn Abi Layla, from 'Abdullah ibn 'Ukaym, he said: The letter of the Prophet, may Allah bless him and grant him peace, came to us while I was a young boy: "Do not make use of the hide of a dead animal, nor its sinews."