Musnad Ahmad 27:1664

Ubayd ibn Abi Qurrah narrated to us, Sulayman—meaning Ibn Bilal—narrated to us, from Abdullah ibn Dinar, from Ibn Umar, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade traveling with the Quran to the land of the enemy, for fear that the enemy might obtain it."