Musnad Ahmad 1082:1

'Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, from Safwan ibn 'Abd Allah, from Umm al-Darda', from Ka'b ibn 'Asim al-Ash'ari—and he was one of the companions of the Saqifah—he said: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "It is not of righteousness to fast while traveling."