Maliks Muwatta Book 21, Hadith Number 7.
Section : Prohibition against Travelling with the Qur’an in Enemy Territory.
Yahya related to me from Malik from Nafi that Abdullah ibn Umar said that the Messenger of Allah, may Allah bless him and grant him peace, forbade travelling with a Qur’an in the land of the enemy. Malik commented, “That is out of fear that the enemy will get hold of it.”