Abu Murawih narrates from Hamzah al-Aslami (may Allah be pleased with him) that he said: "O Messenger of Allah, I have the strength to fast while travelling; is there any sin upon me?" The Messenger of Allah (peace and blessings be upon him) said: "It is a concession from Allah, the Mighty and Majestic. Whoever wishes to take it, that is good; and whoever would like to fast, there is no sin upon him either."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2968