Sahih Muslim Book 007, Hadith Number 3113.
Chapter : What is to be uttered when one sets out for Hajj or any other journey.
Ibn Umar (Allah be pleased with them) reported that whenever Allah's Messenger (may peace be upon him) mounted his camel while setting out on a journey, he glorified Allah (uttered Allah-O-Akbar) thrice, and then said: Hallowed is He Who subdued for us this (ride) and we were not ourselves powerful enough to use It as a ride, and we are going to return to our Lord. O Allah, we seek virtue and piety from Thee in this journey of ours and the act which pleaseth Thee. O Allah, lighten this journey of ours, and make its distance easy for us. O Allah, Thou art (our) companion during the journey, and guardian of (our) family. O Allah, I seek refuge with Thee from hardships of the journey, gloominess of the sights, and finding of evil changes in property and family on return. And he (the Holy Prophet) uttered (these words), and made this addition to them: We are returning, repentant, worshipping our Lord and praising Him.