"The young should greet the old,

  Bismillahirrahmanirrahim

Narrated Abu Huraira Radhi Allahu Anhu : The Prophet Sal-Allahu Alaihi Wasallam said, "The young should greet the old, the passer by should greet the sitting one, and the small group of persons should greet the large group of persons. "

{Sahih Bukhari, Vol 8, Book 74, No. 250}

Narrated Anas bin Malik Radhi Allahu Anhu : that he passed by a group of boys and greeted them and said, "The Prophet Sal-Allahu Alaihi Wasallam used to do so."

{Sahih Bukhari, Vol 8, Book 74, No. 264}