Sahih Muslim Book 026, Hadith Number 5374.

Chapter : The rider should first greet the pedestrian and the small number should greet the large number of persons.

Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: The rider should first greet the pedestrian, and the pedestrian the one who is seated and a small group should greet a larger group (with as-Salam-u-'Alaikum).