Reference : Hisn al-Muslim 214

قَالَ جَابِرٌ رضي الله عنه: 
(كُنَّا إِذَا صَعَدْنَا كَبَّرْنَا، 
وَإِذَا نَزَلْنَا سَبَّحْنَا)
--

Jabir (ra), said: Whenever we went up a hill we would say 
Allāhu Akbar (Allah is the Most Great) 
and when we descended we would say 
Subḥānallāh (Glory is to Allah).

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

Chapter: Takbir and Tasbih during travel