Musnad Abu Ya'la al-Mawsili 149:146

Abu al-Rabi' narrated to us, Hammad narrated to us, Hisham ibn 'Urwah narrated to us, from his father, from 'Aishah: That Hamzah ibn 'Amr al-Aslami asked the Prophet, peace and blessings of Allah be upon him, and said: "O Messenger of Allah, I am a man who fasts continuously; should I fast while traveling?" He said: "Fast if you wish, and break your fast if you wish."