Maliks Muwatta Book 54, Hadith Number 34.

Malik related to me that he heard that when the Messenger of Allah, may Allah bless him and grant him peace, set foot in the stirrup intending to travel, he would say, “In the name of Allah. O Allah! You are my companion in the journey and the Khalifa of my family. O Allah! Spread out the earth for us and make the journey easy for us. O Allah! I seek refuge with You from the hardship of the journey and from returning to sorrow and a distressing sight regarding property and family.”

Malik related to me from a reliable source of his from Yaqub ibn Abdullah al-Ashajj from Bushr ibn Said from Sad ibn Abi Waqqas from Khawla bint Hakim that the Messenger of Allah, may Allah bless him and grant him peace, said, “Whoever dismounts to rest in a place should say, ‘I seek refuge with the complete words of Allah from the evil of what he created,’ (audhu bi kalimati-llahi at-tammati min sharri ma khalaqa), and nothing will harm him until he remounts.”