Musnad Ahmad Hadith 14529

It is narrated from Jabir (may Allah be pleased with him) that once we were with the Prophet (peace and blessings of Allah be upon him) on a military expedition during the month of Ramadan. One companion kept a fast, due to which he became extremely weak and was close to dying of thirst, and his camel began to wander into the bushes. When the Prophet (peace and blessings of Allah be upon him) learned of this, he said: "Bring him to me," and ordered him to break his fast. He said: "Is it not sufficient for you that you have set out in the path of Allah with the Messenger of Allah, and yet you still keep the fast?" So he broke his fast.