Musnad Ahmad 25:1192

Muhammad ibn Mus'ab narrated to us, al-Awza'i narrated to us, from al-Zuhri, from 'Ubayd Allah, from Ibn 'Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) passed by a dead sheep and said: "Why did you not make use of its skin?" They said: "O Messenger of Allah, it is dead (carrion)." He said: "Only its eating has been forbidden."