Musnad Abu Ya'la al-Mawsili 108:116

Sufyan ibn Waki' ibn al-Jarrah narrated to us, my father narrated to us, from Ibrahim ibn Yazid al-Makki, 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 (may Allah bless him and grant him peace) passed by them, and they said: "He is fasting." The Prophet (may Allah bless him and grant him peace) said: "It is not of righteousness to fast while traveling."