(47) باب الْعِلَّةِ الَّتِي مِنْ أَجْلِهَا قِيلَ ذَلِكَ وَذِكْرِ الاِخْتِلاَفِ عَلَى مُحَمَّدِ بْنِ عَبْدِ الرَّحْمَنِ فِي حَدِيثِ جَابِرِ بْنِ عَبْدِ اللَّهِ فِي ذَلِكَ
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."
Reference: Sunan an-Nasa'i 2257
In-book reference: Book 22, Hadith 168
English translation: Vol. 3, Book 22, Hadith 2259