Abu Salamah bin Abd al-Rahman narrates that Hamzah bin Amr 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: "Do whichever of the two is easier for you."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2975