Sahih Muslim Book 6, Hadith Number 2493.
Chapter : Choice for observing fast and breaking it on a journey.
Abu Darda’ reported: We were with the Messenger of Allah (may peace be upon him) 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 (may peace be upon him) and Abdullah b. Rawaha