Musnad Ahmad 33:81

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Muhammad ibn ‘Abd al-Rahman ibn Sa‘d ibn Zurarah al-Ansari, from Muhammad ibn ‘Amr ibn al-Hasan ibn ‘Ali, from Jabir ibn ‘Abd Allah, who said: The Messenger of Allah (may Allah bless him and grant him peace) was—Abu al-Nadr, meaning Hashim, said: "on a journey." Yazid, meaning Ibn Harun, said: "While the Messenger of Allah (may Allah bless him and grant him peace) was on a journey, he saw a man whom the people had gathered around, and he was being shaded." They said: "This is a fasting man." The Messenger of Allah (may Allah bless him and grant him peace) said: "It is not righteousness that you fast while traveling."