Musannaf Ibn Abi Shayba 16:1046

Abu Bakr narrated to us, he said: Jarir narrated to us, from Mansur, from al-Hakam, from Abd al-Rahman ibn Abi Layla, from Abdullah ibn Ukaym, he said: The letter of the Messenger of Allah (may Allah bless him and grant him peace) came to us: "Do not make use of the dead animal (maytah) for either skin (ihab) or sinew (asab)."