Sahih Muslim
The Book on Government (Kitab Al-Imara)
Book Number: 020 / Hadith Number: 4608
It has been narrated on the authority of Abdullah b. Umar that the Messenger of Allah (may peace be upon him) used to forbid that one should travel to the land of the enemy taking the Qur'an (with him) lest it should fall into the hands of the enemy.