Musnad Abu Ya'la al-Mawsili 156:11

Zakariya ibn Yahya narrated to us, Rawh narrated to us, Habib 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 the few should greet the many."