Sahih Muslim Book 7, Hadith Number 3116.

Chapter : What is to be recited on return from the journey of Hajj or any other journey.

Abdullah b. ‘Umar reported that whenever Allah’s Messenger (may peace be upon him) came back from the battle or from expeditions or from Hajj or Umra and as he reached the top of the hillock or upon the elevated hard ground, he uttered Allah-O-Akbar thrice, and then said: There is no god but Allah. He is One, there is no partner with Him, His is the sovereignty and His is the praise and He is Potent over everything. (We are) returning, repenting, worshipping, prostrating before our Lord, and we praise Him Allah fulfilled His promise and helped His servant, and routed the confederates alone.