Musnad Ahmad 262:4

Muhammad ibn Ja'far narrated to us, he said: Sa'id narrated to us, from Qatadah, from Sulayman ibn Yasar, from Hamzah ibn 'Amr al-Aslami, that he asked the Messenger of Allah, peace and blessings of Allah be upon him, about fasting while traveling. He said: "If you wish, you may fast, and if you wish, you may break your fast."