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." (Using translation from Nasāʾī 2257)
أَنَّ رَسُولَ اللهِ ﷺ فِي بَعْضِ أَسْفَارِهِ رَأَى نَاسًا مُجْتَمِعِينَ عَلَى رَجُلٍ فَسَأَلَ رَسُولُ اللهِ ﷺ فقَالَوا رَجُلٌ جَهَدَهُ الصِّيَامُ فَقَالَ رَسُولُ اللهِ ﷺ لَيْسَ الْبِرُّ الصِّيَامَ فِي السَّفَرِ