Sahih Muslim — The Book on Government — Hadith No: 4840
وَحَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا لَيْثٌ، ح وَحَدَّثَنَا ابْنُ رُمْحٍ، أَخْبَرَنَا اللَّيْثُ، عَنْ نَافِعٍ، عَنْ عَبْدِ، اللَّهِ بْنِ عُمَرَ عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم أَنَّهُ كَانَ يَنْهَى أَنْ يُسَافَرَ بِالْقُرْآنِ إِلَى أَرْضِ الْعَدُوِّ مَخَافَةَ أَنْ يَنَالَهُ الْعَدُوُّ .
Translation
It has been narrated on the authority of Abdullah b. Umar that the Messenger of Allah (ﷺ) 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