Musnad Ahmad 1153:1647

Yahya narrated to us, he said: Hisham ibn Urwah narrated to us, he said: My father informed me, from Aisha, she said: Hamza ibn Amr al-Aslami came to the Messenger of Allah, may Allah bless him and grant him peace, and said: "I used to fast," meaning I would fast continuously. "Should I fast while traveling?" He said: "If you wish, then fast, and if you wish, then do not fast."