Musnad Ahmad 30:2082

Ali ibn Ishaq narrated to us, Abdullah and 'Attab narrated to us, they said: Abdullah narrated to us, he said: Shu'bah informed us, from a certain man of the Khath'am tribe, that he heard Abu Zur'ah narrating from Abu Hurairah, that the Prophet, peace and blessings of Allah be upon him, when he would set out on a journey and mount his riding beast, he would say: "O Allah, You are the Companion in the journey and the Successor (guardian) over the family." He said: And I think he said: "And the One who carries upon the back (the mount). O Allah, accompany us with counsel (goodness), and return us with a pledge (of safety). We seek refuge in You from the hardships of the journey and the gloominess of the return."