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.