Sulayman bin Yasar narrates that Hamzah al-Aslami (may Allah be pleased with him) asked the Messenger of Allah (peace and blessings be upon him) about fasting while travelling, and he (peace and blessings be upon him) said: "If you wish, you may fast; and if you wish, you may break your fast."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2971