Musnad Ahmad 359:1

Uthman ibn Umar narrated to us, he said: Malik ibn Anas informed us, from Sumayy, from Abu Bakr ibn Abd al-Rahman ibn al-Harith, from a man among 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, fasted during a journey in the year of the Conquest, and he ordered his companions to break their fast." And he said: "You are going to encounter your enemy, so gain strength." It was said: "O Messenger of God, the people have fasted because of your fasting." So when he reached al-Kadeed, he broke his fast. The one who narrated to me said: "I saw the Messenger of God, may God bless him and grant him peace, pouring water over his head due to the heat while he was fasting."