Musnad Ahmad 33:311

Yahya ibn Sa'id narrated to us, from Shu'bah, Muhammad ibn 'Abd al-Rahman narrated to us, from Muhammad ibn 'Amr ibn al-Hasan, from Jabir ibn 'Abd Allah, that the Messenger of Allah (may Allah bless him and grant him peace) was on a journey, and he saw a man who was crowded around and shaded. He said, "What is this?" They said, "He is fasting." He said, "It is not righteousness to fast—or righteousness for the fasting person—while traveling."