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
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا أَبُو خَيْثَمَةَ، عَنْ حُمَيْدٍ، قَالَ سُئِلَ أَنَسٌ - رضى الله عنه - عَنْ صَوْمِ رَمَضَانَ فِي السَّفَرِ فَقَالَ سَافَرْنَا مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فِي رَمَضَانَ فَلَمْ يَعِبِ الصَّائِمُ عَلَى الْمُفْطِرِ وَلاَ الْمُفْطِرُ عَلَى الصَّائِمِ .
Humaid reported that Anas (Allah be pleased with him) was asked about fasting during Ramadan while travelling. He said: We travelled with the Messenger of Allah (ﷺ) during the month of Ramadan, but neither the observer of the fast found fault with the breaker of the fast, nor the breaker of the fast found fault with the observer of the fast.
Reference : Sahih Muslim 1118aIn-book reference : Book 13, Hadith 126USC-MSA web (English) reference : Book 6, Hadith 2482