Muwatta Malik 965

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّهُ قَالَ نَهَى رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ يُسَافَرَ بِالْقُرْآنِ إِلَى أَرْضِ الْعَدُوِّ ‏.‏

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

Reference

Muwatta Malik 965

In-book

Book 21, Hadith 7