Nafi' narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) "forbade travelling with the Qur'an to the territory of the enemy, out of fear that the enemy might take it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18241