Musnad Ahmad 788:62

Abu Ahmad narrated to us, Burayd ibn Abdullah narrated to us, Abu Burdah narrated to us, from Abu Musa, who said: "Maintain this Quran. By the One in whose hand is my soul, it is more prone to escaping than one of you [is prone to losing] a camel from its hobble." Abu Ahmad said: I said to Burayd, "Are these the hadiths that you narrated to me from Abu Burdah, from Abu Musa, from the Prophet, may Allah bless him and grant him peace?" He said: "They are from the Prophet, may Allah bless him and grant him peace, but I do not say to you..."