Musnad Abu Ya'la al-Mawsili 109:204

Sufyan ibn Waki' narrated to us, my father narrated to us, from Ibrahim ibn Yazid, from Abu al-Zubayr, from Jabir, who said: We were on a journey, and a man fasted, then he fainted. His companions stood around him. The Prophet, peace and blessings of Allah be upon him, passed by and they said: "He is fasting." The Prophet, peace and blessings of Allah be upon him, said: "It is not of righteousness to fast while traveling."