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 1115 c

This hadith has been narrated on the authority of Shu'ba with the same chain of transmitters but with this addition that he (the Holy Prophet) said:

" Take advantage of the concession of Allah Who Wanted it to you." When he (one of the narrators) asked him (the other one, Yabya b. Abi Kathar) he did not retain it in his mind.
وَحَدَّثَنَاهُ أَحْمَدُ بْنُ عُثْمَانَ النَّوْفَلِيُّ، حَدَّثَنَا أَبُو دَاوُدَ، حَدَّثَنَا شُعْبَةُ، بِهَذَا الإِسْنَادِ ‏.‏ نَحْوَهُ وَزَادَ قَالَ شُعْبَةُ وَكَانَ يَبْلُغُنِي عَنْ يَحْيَى بْنِ أَبِي كَثِيرٍ أَنَّهُ كَانَ يَزِيدُ فِي هَذَا الْحَدِيثِ وَفِي هَذَا الإِسْنَادِ أَنَّهُ قَالَ ‏ "‏ عَلَيْكُمْ بِرُخْصَةِ اللَّهِ الَّذِي رَخَّصَ لَكُمْ ‏"‏ ‏.‏ قَالَ فَلَمَّا سَأَلْتُهُ لَمْ يَحْفَظْهُ ‏.‏
Reference : Sahih Muslim 1115c
In-book reference : Book 13, Hadith 120
USC-MSA web (English) reference : Book 6, Hadith 2476
  (deprecated numbering scheme)