Musnad Ahmad 30:3493

Rawh narrated to us, Ibn Jurayj and Abdullah ibn al-Harith narrated to us, from Ibn Jurayj, he said: Ziyad informed me that Thabit, the freed slave of Abd al-Rahman ibn Zayd, informed him that he heard Abu Hurayrah say: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The rider should greet the walker, the walker should greet the one who is sitting, and the few should greet the many."