Musnad Ahmad 26:379

Aswad ibn Amir narrated to us, Isra'il narrated to us, from Abu Ishaq, from Khumayr ibn Malik, who said: It was ordered that the copies of the Qur'an (musahif) be changed. He said: Ibn Mas'ud said: "Whoever among you is able to hide (ghalla) his copy of the Qur'an, let him hide it, for whoever hides something will bring it with him on the Day of Resurrection." He said: Then he said: "I recited seventy surahs from the mouth of the Messenger of Allah, may Allah bless him and grant him peace. Shall I then abandon what I took from the mouth of the Messenger of Allah, may Allah bless him and grant him peace?"