Urwah bin al-Zubayr narrates that Hamzah bin Amr al-Aslami (may Allah be pleased with him) asked the Prophet (peace and blessings be upon him) about fasting while travelling, and he (peace and blessings be upon him) said: "Fast if you wish, and break your fast if you wish."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2967