Musnad Ahmad 1082:2

'Abd al-Razzaq and Ibn Bakr narrated to us, they said: Ibn Jurayj narrated to us, he said: Ibn Shihab narrated to me, that Safwan ibn 'Abd Allah ibn Safwan narrated to him, from Umm al-Darda', from Ka'b al-Ash'ari—Ibn Bakr ibn 'Asim said: That the Messenger of Allah, peace and blessings of Allah be upon him, said: "It is not of righteousness to fast while traveling."