Musnad Ahmad 211:1

Ishaq ibn Isa narrated to us, he said: Malik informed me, from Sumayy, from Abu Bakr ibn Abd al-Rahman ibn al-Harith ibn Hisham, from one of the companions of the Prophet, may God bless him and grant him peace: That the Messenger of God, may God bless him and grant him peace, commanded the people to break their fast in the year of the Conquest, and he said: "Gain strength for your enemy," and the Messenger of God, may God bless him and grant him peace, fasted. Abu Bakr said: The one who narrated to me said: I saw the Messenger of God, may God bless him and grant him peace, at al-Arj, "pouring water over his head due to thirst" — or due to the heat. Then it was said: "O Messenger of God, a group of people have fasted when you fasted." So when he was at al-Kadid, "he called for a cup and drank," and the people broke their fast.