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 1116 a

Abu Sa'id al-Khudri (Allah be pleased with him) reported:

We went out on an expedition with Allah's Messenger (ﷺ) on the 16th of Ramadan. Some of us fasted and some of us broke the fast. But neither the observer of the fast found fault with one who broke it, nor the breaker of the fast found fault with one who observed it.
حَدَّثَنَا هَدَّابُ بْنُ خَالِدٍ، حَدَّثَنَا هَمَّامُ بْنُ يَحْيَى، حَدَّثَنَا قَتَادَةُ، عَنْ أَبِي نَضْرَةَ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، - رضى الله عنه - قَالَ غَزَوْنَا مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم لِسِتَّ عَشْرَةَ مَضَتْ مِنْ رَمَضَانَ فَمِنَّا مَنْ صَامَ وَمِنَّا مَنْ أَفْطَرَ فَلَمْ يَعِبِ الصَّائِمُ عَلَى الْمُفْطِرِ وَلاَ الْمُفْطِرُ عَلَى الصَّائِمِ ‏.‏
Reference : Sahih Muslim 1116a
In-book reference : Book 13, Hadith 121
USC-MSA web (English) reference : Book 6, Hadith 2477
  (deprecated numbering scheme)