Musannaf Ibn Abi Shayba 28:1101

Ibn Numayr narrated to us, he said: Ubayd Allah ibn Umar narrated to us, from Nafi, from Ibn Umar, from the Prophet (may Allah bless him and grant him peace), that he used to say when he returned from a military expedition, or a raiding party, or Hajj, or Umrah: Whenever he ascended a mountain pass or a high plateau, he would say "Allahu Akbar" (God is the Greatest) three times. Then he would say: "There is no god but Allah alone. He fulfilled His promise, and He helped His servant, and He defeated the confederates alone. We are returning, repenting, worshipping, and praising our Lord."