Jaabir (radhi-yAllaahu 'anhu) said: While ascending, we would say:

اَللّٰهُ أَكْبَرُ

Allah is the Most Great

Allaahu 'Akbar

And when we descended we would say:

سُبْحَانَ اللّٰهِ

Glory is to Allah.

Subhaanallaah