The choice between fasting and not fasting when travelling Sahih Muslim 1122 b
Translation
Abu Darda' reported
We were with the Messenger of Allah (ﷺ) on some of his journeys on an intensely hot day so much so that a person would place his hand on his head (in order to protect himself) against excessive heat, and none amongst us was fasting but the Messenger of Allah (ﷺ) and Abdullah b. Rawaha