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
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 .
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.
Reference : Sahih Muslim 1115cIn-book reference : Book 13, Hadith 120USC-MSA web (English) reference : Book 6, Hadith 2476