Musnad Ahmad 33:393

Zayd ibn al-Hubab narrated to us, Husayn ibn Waqid narrated to me, from Abu al-Zubayr, who said: I heard Jabir saying: The Prophet, peace and blessings of Allah be upon him, passed by a man who was turning his back to his stomach [due to weakness from hunger]. He asked about him, and they said: "He is fasting, O Prophet of Allah." So he called him and ordered him to break his fast. He said: "Is it not enough for you to be in the way of Allah, and with the Messenger of Allah, peace and blessings of Allah be upon him, that you must also fast?"