Sahih Muslim Book 6, Hadith Number 2466.
Chapter : Permissibility of observing the fast or not observing it in the month of Ramadan for a traveller.
Ibn Abbas (Allah be pleased with him) reported that the Messenger of Allah (may peace be upon him) went out during the month of Ramadan in the year of Victory (when Mecca was conquered) and was fasting till he reached Kadid (a canal situated at a distance of forty-two miles from Mecca) and he then broke the fast. And it was the habit of the Companions of the Messenger of Allah (may peace be upon him) to follow him in every new thing (or act). So they followed him also (in this matter).