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 c

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

We went out on an expedition with the Messenger of Allah (ﷺ) during Ramadan and neither the observer of the fast was found fault with for his fasting, nor the breaker of the fast for breaking it.
حَدَّثَنَا نَصْرُ بْنُ عَلِيٍّ الْجَهْضَمِيُّ، حَدَّثَنَا بِشْرٌ، - يَعْنِي ابْنَ مُفَضَّلٍ - عَنْ أَبِي مَسْلَمَةَ، عَنْ أَبِي نَضْرَةَ، عَنْ أَبِي سَعِيدٍ، - رضى الله عنه - قَالَ كُنَّا نُسَافِرُ مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فِي رَمَضَانَ فَمَا يُعَابُ عَلَى الصَّائِمِ صَوْمُهُ وَلاَ عَلَى الْمُفْطِرِ إِفْطَارُهُ ‏.‏
Reference : Sahih Muslim 1116c
In-book reference : Book 13, Hadith 123
USC-MSA web (English) reference : Book 6, Hadith 2479
  (deprecated numbering scheme)