Sahih Muslim 1117
Sahih Muslim · Book 13: The Book of Fasting · 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
Abu Nadra reported Abu Sa'id al. Khudri and Jabir b. Abdullah as saying:
حَدَّثَنَا سَعِيدُ بْنُ عَمْرٍو الأَشْعَثِيُّ ، وَسَهْلُ بْنُ عُثْمَانَ ، وَسُوَيْدُ بْنُ سَعِيدٍ ، وَحُسَيْنُ، بْنُ حُرَيْثٍ كُلُّهُمْ عَنْ مَرْوَانَ ، - قَالَ سَعِيدٌ أَخْبَرَنَا مَرْوَانُ بْنُ مُعَاوِيَةَ، - عَنْ عَاصِمٍ ، قَالَ سَمِعْتُ أَبَا نَضْرَةَ ، يُحَدِّثُ عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ ، وَجَابِرِ بْنِ عَبْدِ اللَّهِ ، - رضى الله عنهم - قَالاَ سَافَرْنَا مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فَيَصُومُ الصَّائِمُ وَيُفْطِرُ الْمُفْطِرُ فَلاَ يَعِيبُ بَعْضُهُمْ عَلَى بَعْضٍ .
We travelled with the Messenger of Allah (ﷺ). The observer of the fast observed it, and the breaker of the fast broke it, but none of them found fault with each other.