Mukhtasar Bukhari

Narrated Jabir bin Abdullah (may Allah be pleased with him): The Messenger of Allah (peace and blessings of Allah be upon him) was on a journey (the expedition of the Conquest), and he saw a crowd and a man over whom shade was being provided. He (the Prophet) said: "What has happened to him?" The people replied: "He is fasting." So the Prophet (peace be upon him) said: "It is not good to fast while traveling (in this condition)." (Because in this matter, there is an option to fast or not to fast. See the previous hadith in the chapter: Fasting and breaking the fast during travel.)