Sahih Muslim 1118
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 Khalid al-Ahmar narrated from Humaid who said:
وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، حَدَّثَنَا أَبُو خَالِدٍ الأَحْمَرُ ، عَنْ حُمَيْدٍ ، قَالَ خَرَجْتُ فَصُمْتُ فَقَالُوا لِي أَعِدْ . قَالَ فَقُلْتُ إِنَّ أَنَسًا أَخْبَرَنِي أَنَّ أَصْحَابَ رَسُولِ اللَّهِ صلى الله عليه وسلم كَانُوا يُسَافِرُونَ فَلاَ يَعِيبُ الصَّائِمُ عَلَى الْمُفْطِرِ وَلاَ الْمُفْطِرُ عَلَى الصَّائِمِ . فَلَقِيتُ ابْنَ أَبِي مُلَيْكَةَ فَأَخْبَرَنِي عَنْ عَائِشَةَ - رضى الله عنها - بِمِثْلِهِ .
I went out and was fasting; they said to me: Break (lit. go back, repeat). He said that Anas reported that the Companions of the Messenger of Allah (ﷺ) used to set out on a journey and 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. (One of the narrators Humaid said): I met Ibn Abi Mulaika who informed me the same thing on the authority of 'A'isha.