Musnad Ahmad 1153:1712

Waki' narrated to us, Hisham narrated to us, from his father, from Aisha, who said: Hamza al-Aslami asked the Messenger of Allah, peace and blessings of Allah be upon him, about fasting while traveling. He was a man who used to fast continuously. He said: "You have the choice; if you wish, then fast, and if you wish, then break your fast."