Sayyiduna Ammar bin Yasir (may Allah be pleased with him) narrated: We were returning with the Prophet (peace and blessings be upon him) from an expedition. We kept travelling on a severely hot day, and we made camp along the way. A man from among us went and came into the shade of a tree. His companions were supporting him; he was lying down, and he seemed to be in pain. When the Prophet (peace and blessings be upon him) saw (this), he asked: What is the matter with your companion? The people said: He is fasting. The Prophet (peace and blessings be upon him) said: It is not righteousness that you fast during travel. It is binding on you to adopt the concession that Allah Almighty has granted you; accept it.
Imam al-Tabarani recorded this narration in al-Mu'jam al-Kabir, and its chain is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 4929