Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.148.

Jabir said, “We used to say Allahu akbar when we went up a high place, and subhanallah when we went down a valley.” (Bukhari)

Ibn ‘Umar reported that the Prophet, peace be upon him, returned from hajj (or ‘umrah). When he approached a path high up in mountain or one that was difficult, he would back up three steps and say. “There is no god but Allah, He is One. He has no partners. To Him belongs all dominion and praise, and He has power over all things. We are returning repentant, worshipping, prostrating, praising our Lord. He kept His promise. And gave victory to His servant, and defeated all the confederates alone.”