Musnad Ahmad 27:1851

Abu Kamil narrated to us, Hammad—meaning Ibn Salamah—narrated to us, from Abu al-Zubayr, from Ali ibn Abdullah al-Bariqi, from Abdullah ibn Umar, that the Prophet, peace and blessings of Allah be upon him, when he would mount his riding beast, he would say "Allahu Akbar" (Allah is the Greatest) three times. Then he would say: "Exalted is He who has subjected this to us, and we could not have [otherwise] subdued it. And indeed we are, to our Lord, about to return." [Az-Zukhruf: 13-14]. Then he would say: "O Allah, I ask You in this journey of mine for righteousness and piety, and of deeds that which You are pleased with. O Allah, make the journey easy for us, and fold for us its distance. O Allah, You are the Companion in the journey and the Successor over the family. O Allah, accompany us in our journey and be the Successor over our family." And when he would return to his family, he would say: "Returning, repenting, if Allah wills, worshipping, to our Lord praising."