The Book on Government
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 .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
From Sahih Muslim, whose reports the scholars accept as authentic.