Sahih Muslim Book 6, Hadith Number 2491.

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

Hamza b. ‘Amr al-Aslami (Allah be pleased with him) said: Messenger of Allah, I find strength in me for fasting on a journey; is there any sin upon me (in doing it)? Thereupon the Messenger of Allah (may peace be upon him) said: It is a concession from Allah. He who took advantage of it, it is good for him, and he who preferred to observe fast, there is no sin upon him. Harun (one of the narrators) in his narration said: “It is a concession, and he made no mention of ‘from Allah'”.