Sahih Muslim Book 6, Hadith Number 2492.

Chapter : Choice for observing fast and breaking it on a journey.

Abu Darda’ (Allah be pleased with him) reported: We set out during the month of Ramadan with the Messenger of Allah (may peace be upon him) in such an intense heat that one of us would place his hand over his head (in order to protect himself) against the excessive heat, and none among us was observing the fast, except the Messenger of Allah (may peace be upon him) and ‘Abdullah b. Rawaha.