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

It has been narrated on the authority of Ibn 'Umar who said:

The Messenger of Allah (ﷺ) forbade that one should travel to the land of the enemy taking the Qur'an with him.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، قَالَ نَهَى رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ يُسَافَرَ بِالْقُرْآنِ إِلَى أَرْضِ الْعَدُوِّ ‏.‏
Reference : Sahih Muslim 1869 a
In-book reference : Book 33, Hadith 135
USC-MSA web (English) reference : Book 20, Hadith 4607
  (deprecated numbering scheme)