Musnad Ahmad 30:3494

Rawh narrated to us, Habib—meaning Ibn al-Shahid—narrated to us, from al-Hasan, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The rider should greet the walker, and the walker should greet the one who is sitting." And he said in Baghdad: "And the few should greet the many, and the young should greet the old." And Rawh said in Baghdad: "The few should greet the many."