Riyad as-Salihin 7:21

Ibn 'Umar (May Allah be pleased with them) reported:

Whenever the Prophet (ﷺ) and his army ascended a height, they would proclaim: "Allahu Akbar (Allah is Greatest)," and when they climbed down, they would proclaim: "Subhan Allah (Allah is free from imperfection)." [Abu Dawud] .