Musannaf Ibn Abi Shayba 21:494

Abdullah ibn Numayr narrated to us, Ubaydullah ibn Umar narrated to us, from Nafi, from Ibn Umar, from the Prophet (may Allah bless him and grant him peace), that when he returned from a military expedition, a raiding party, Hajj, or Umrah, he would say whenever he ascended a mountain pass or a high plateau: "Allah is the Greatest" three times. Then he would say: "There is no god but Allah alone; He fulfilled His promise, we are repentant, worshippers, and praisers to our Lord."