كتاب الصيام
13  
The Book of Fasting
(15)
Chapter: It is permissible to fast or not to fast during Ramadan for one who is travelling for no sinful purpose, if his journey is two stages or further. But it is better for the one who is able to fast without suffering any harm to do so, and the one for whom it is difficult may break the fast
(15)
باب جواز الصوم والفطر في شهر رمضان للمسافر في غير معصية إذا كان سفره مرحلتين فأكثر وأن الأفضل لمن أطاقه بلا ضرر أن يصوم ولمن يشق عليه أن يفطر
Sahih Muslim 1113 c

It has been narrated on the authority of Zuhri with the same chain of transmitters that breaking of fast (in a journey) is the final of the two commands (whether one may fast or one may break it), and it is the last command of the Messenger of Allah (ﷺ) which is to be accepted as final. Zuhri said:

The Messenger of Allah (ﷺ) marched on Mecca on the morning of 14th of Ramadan (lit. when thirteen nights had passed).
حَدَّثَنِي مُحَمَّدُ بْنُ رَافِعٍ، حَدَّثَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، عَنِ الزُّهْرِيِّ، بِهَذَا الإِسْنَادِ قَالَ الزُّهْرِيُّ وَكَانَ الْفِطْرُ آخِرَ الأَمْرَيْنِ وَإِنَّمَا يُؤْخَذُ مِنْ أَمْرِ رَسُولِ اللَّهِ صلى الله عليه وسلم بِالآخِرِ فَالآخِرِ ‏.‏ قَالَ الزُّهْرِيُّ فَصَبَّحَ رَسُولُ اللَّهِ صلى الله عليه وسلم مَكَّةَ لِثَلاَثَ عَشْرَةَ لَيْلَةً خَلَتْ مِنْ رَمَضَانَ ‏.‏
Reference : Sahih Muslim 1113c
In-book reference : Book 13, Hadith 112
USC-MSA web (English) reference : Book 6, Hadith 2468
  (deprecated numbering scheme)