Sunan an-Nasa'i, Volume 3, Book 22, Hadith 2259 (DarusSalam) ⓘ

It was narrated from Jabir bin 'Abdullah that: the Messenger of Allah saw some people gathered around a man, so he asked (what was happening) and they said: "It is a man who is exhausted because of fasting." The Messenger of Allah said: "It is not righteousness to fast when traveling."

أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا بَكْرٌ، عَنْ عُمَارَةَ بْنِ غَزِيَّةَ، عَنْ مُحَمَّدِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ جَابِرِ بْنِ عَبْدِ اللَّهِ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم رَأَى نَاسًا مُجْتَمِعِينَ عَلَى رَجُلٍ فَسَأَلَ فَقَالُوا رَجُلٌ أَجْهَدَهُ الصَّوْمُ ‏.‏ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ لَيْسَ مِنَ الْبِرِّ الصِّيَامُ فِي السَّفَرِ ‏" ‏ ‏.‏