Sahih Muslim Book 6, Hadith Number 2468.
Chapter : Permissibility of observing the fast or not observing it in the month of Ramadan for a traveller.
It has been narrated on the authority of Zuhri with the same chain of transmitters that breaking of fast (in a journey) is the final of the two commands (whether one may fast or one may break it), and it is the last command of the Messenger of Allah (may peace be upon him) which is to be accepted as final. Zuhri said: The Messenger of Allah (may peace be upon him) marched on Mecca on the morning of 14th of Ramadan (lit when thirteen nights had passed).