Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with the same chain of narration, it is narrated from (Sayyiduna Abdullah bin Umar, may Allah be pleased with him) that the Messenger of Allah, peace and blessings be upon him, forbade traveling with the Qur'an to the lands of the enemies (of Islam). Imam Malik, may Allah have mercy on him, said: I think the reason for this is the fear that it might fall into the hands of the enemy.