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 1114 b

This hadith has been narrated by Ja'far with the same chain of transmitters and he added:

It was said to him (to the Holy Prophet): There are people to whom fasting has become unbearable and they are waiting how you do. He (the Holy Prophet) then called for a cup of water when it was afternoon. The rest of the hadith is the same.
وَحَدَّثَنَاهُ قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا عَبْدُ الْعَزِيزِ، - يَعْنِي الدَّرَاوَرْدِيَّ - عَنْ جَعْفَرٍ، بِهَذَا الإِسْنَادِ وَزَادَ فَقِيلَ لَهُ إِنَّ النَّاسَ قَدْ شَقَّ عَلَيْهِمُ الصِّيَامُ وَإِنَّمَا يَنْظُرُونَ فِيمَا فَعَلْتَ ‏.‏ فَدَعَا بِقَدَحٍ مِنْ مَاءٍ بَعْدَ الْعَصْرِ ‏.‏
Reference : Sahih Muslim 1114b
In-book reference : Book 13, Hadith 117
USC-MSA web (English) reference : Book 6, Hadith 2473
  (deprecated numbering scheme)