كتاب الصيام
13  
The Book of Fasting
(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
(15)
باب جواز الصوم والفطر في شهر رمضان للمسافر في غير معصية إذا كان سفره مرحلتين فأكثر وأن الأفضل لمن أطاقه بلا ضرر أن يصوم ولمن يشق عليه أن يفطر
Sahih Muslim 1116 d

Abu Sa'id al-Khudri (Allah be pleased with him) reported:

We went out on an expedition with the Messenger of Allah (ﷺ) during Ramadan. Some of us observed the fast and some of us broke it. Neither the observer of the fast had any grudge against one who broke it, nor the breaker of the fast had any grudge against one who had fasted They knew that he who had strength enough (to bear its rigour) fasted and that was good, and they also found that he who felt weakness (and could not bear the burden) broke it, and that was also good.
حَدَّثَنِي عَمْرٌو النَّاقِدُ، حَدَّثَنَا إِسْمَاعِيلُ بْنُ إِبْرَاهِيمَ، عَنِ الْجُرَيْرِيِّ، عَنْ أَبِي نَضْرَةَ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، - رضى الله عنه - قَالَ كُنَّا نَغْزُو مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فِي رَمَضَانَ فَمِنَّا الصَّائِمُ وَمِنَّا الْمُفْطِرُ فَلاَ يَجِدُ الصَّائِمُ عَلَى الْمُفْطِرِ وَلاَ الْمُفْطِرُ عَلَى الصَّائِمِ يَرَوْنَ أَنَّ مَنْ وَجَدَ قُوَّةً فَصَامَ فَإِنَّ ذَلِكَ حَسَنٌ وَيَرَوْنَ أَنَّ مَنْ وَجَدَ ضَعْفًا فَأَفْطَرَ فَإِنَّ ذَلِكَ حَسَنٌ ‏.‏
Reference : Sahih Muslim 1116d
In-book reference : Book 13, Hadith 124
USC-MSA web (English) reference : Book 6, Hadith 2480
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