Musnad Abu Ya'la al-Mawsili 150:415

Ubayd Allah ibn Umar al-Qawariri narrated to us, Muhammad ibn Salamah narrated to us, Muhammad ibn Ajlan narrated to us, from Hisham ibn Urwah, from his father, from Aisha, who said: Hamzah ibn Amr al-Aslami asked the Messenger of Allah, may Allah bless him and grant him peace, and said: "O Messenger of Allah, should I fast while traveling?" He said: "If you wish, then fast, and if you wish, then do not fast."