Musnad Ahmad 33:295

Isma'il narrated to us, from Shu'bah, from Muhammad ibn 'Abd al-Rahman ibn Sa'd ibn Zurarah, from Muhammad ibn 'Amr ibn al-Hasan ibn 'Ali, that he heard Jabir ibn 'Abd Allah saying: While the Messenger of Allah (peace and blessings of Allah be upon him) was on a journey, he saw a crowd and a man who had been shaded. He asked about him, and they said: "This is a fasting man." He said: "It is not righteousness that you fast while traveling."