Musnad Ahmad 30:1193

Rawh narrated to us, Ibn Jurayj narrated to us, Ziyad informed me, that Thabit, the freed slave of Abd al-Rahman ibn Zayd, informed him that he heard Abu Hurayrah saying: 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."