Language:
كتاب الجهاد والسير
56  
Fighting for the Cause of Allah (Jihaad)
(132)
Chapter: Reciting Subhan Allah when going down a valley
(132)
باب التَّسْبِيحِ إِذَا هَبَطَ وَادِيًا
Sahih al-Bukhari 2993

Narrated Jabir bin `Abdullah:

Whenever we went up a place we would say, "Allahu--Akbar (i.e. Allah is Greater)", and whenever we went down a place we would say, "Subhan Allah."

حَدَّثَنَا مُحَمَّدُ بْنُ يُوسُفَ، حَدَّثَنَا سُفْيَانُ، عَنْ حُصَيْنِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ سَالِمِ بْنِ أَبِي الْجَعْدِ، عَنْ جَابِرِ بْنِ عَبْدِ اللَّهِ ـ رضى الله عنهما ـ قَالَ كُنَّا إِذَا صَعِدْنَا كَبَّرْنَا، وَإِذَا نَزَلْنَا سَبَّحْنَا‏.‏
Reference : Sahih al-Bukhari 2993
In-book reference : Book 56, Hadith 202
USC-MSA web (English) reference : Vol. 4, Book 52, Hadith 236
  (deprecated numbering scheme)