Yahya related to me from Malik from Zayd ibn Aslam that theMessenger of Allah, may Allah bless him and grant him peace, said,"The one riding greets the one walking, and when one of a group ofpeople gives a greeting, it is enough for all of them."
USC-MSA web (English) reference: Book 53, Hadith 1
Arabic reference: Book 53, Hadith 1759