Musnad Ahmad 482:118

Abu Abd al-Rahman narrated to us, Ibn Lahi'ah narrated to us, Mushrih ibn Ha'an Abu al-Mus'ab al-Mu'afiri narrated to me, he said: I heard Uqbah ibn Amir al-Juhani saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "If the Quran were in a hide, then it were cast into the Fire, it would not burn."