Sahih Bukhari
Sahih Bukhari Volume 008, Book 074, Hadith Number 250.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 74. Asking Permission
Narated By Abu Huraira: The Prophet said, "The young should greet the old, the passerby should greet the sitting one, and the small group of persons should greet the large group of persons."