كتاب الإمارة
33  
The Book on Government
(24)
Chapter: The prohibition of travelling with the Mushaf to the land of the disbelievers if there is the fear that it may fall into their hands.
(24)
باب النَّهْىِ أَنْ يُسَافَرَ بِالْمُصْحَفِ إِلَى أَرْضِ الْكُفَّارِ إِذَا خِيفَ وُقُوعُهُ بِأَيْدِيهِمْ
Sahih Muslim 1869 b

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.

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 ‏.‏
Reference : Sahih Muslim 1869b
In-book reference : Book 33, Hadith 136
USC-MSA web (English) reference : Book 20, Hadith 4608
  (deprecated numbering scheme)