It is narrated on the authority of 'Abdullah ibn' Umar (RA) that the Messenger of Allah (ﷺ) used to rise on every high ground on the way back from a battle or Hajj or 'Umrah and say three times:

اَللَّهُ أَكْبَرُ

Allahu Akbar

Allah is the Most Great.

Reference: Bukhari: 1797