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

اللَّهُ أَكْبَرُ

Allah is the Most Great

Allaahu 'Akbar

And when we descended we would say:

سُبْحَانَاللَّهِ

Glory is to Allah.

Subhaanallaah

Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 6/135.