Sahih Muslim Book 6, Hadith Number 2474.

Chapter : Permissibility of observing the fast or not observing it in the month of Ramadan for a traveller.

Jabir b. ‘Abdullah (Allah be pleased with both of them) reported that in the course of a journey Allah’s Messenger (may peace be upon him) saw a man, people crowding around him and providing him a shade. Upon this he (the Holy Prophet) said: What is the matter with him? They said: He is a person observing fast. Whereupon the Messenger of Allah (may peace be upon him) said: It is no righteousness that you fast on journey.