كَانَ النَّبِيُّ ﷺ إِذَا عَلَوْا كَبَّرُوا وَإِذَا نَزَلُوا سَبَّحُوا — اللَّهُ أَكْبَرُ، سُبْحَانَ اللَّهِ
Transliteration
Allahu Akbar (when ascending), Subhanallah (when descending)
Translation
When the Prophet (peace be upon him) and his companions ascended they would say "Allah is the Greatest", and when they descended they would say "Glory be to Allah".
Reference: Abu Dawud 2599