Musnad Ahmad 33:453

Rawh narrated to us, Ash'ath narrated to us, from al-Hasan, from Jabir ibn Abdullah, who said: "We used to travel with the Prophet, peace and blessings of Allah be upon him. When we ascended, we would proclaim the greatness of Allah (say 'Allahu Akbar'), and when we descended, we would declare the perfection of Allah (say 'Subhan Allah')."