The Book of Fasting
13/15. 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
Sahih Muslim
وَحَدَّثَنِي حَرْمَلَةُ بْنُ يَحْيَى، أَخْبَرَنَا ابْنُ وَهْبٍ، أَخْبَرَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، بِهَذَا الإِسْنَادِ مِثْلَ حَدِيثِ اللَّيْثِ . قَالَ ابْنُ شِهَابٍ فَكَانُوا يَتَّبِعُونَ الأَحْدَثَ فَالأَحْدَثَ مِنْ أَمْرِهِ وَيَرَوْنَهُ النَّاسِخَ الْمُحْكَمَ .
A hadlth like this has been transmitted on the authority of Ibn Shibab who said that they (the Compnions of the Holy Prophet) followed the latest of his commands and looked upon it as one abrogating (the previous ones) and the most firm.
Reference : Sahih Muslim 1113dIn-book reference : Book 13, Hadith 113USC-MSA web (English) reference : Book 6, Hadith 2469