Musnad Ahmad 1118:15

Hassan ibn Musa narrated to us, Ibn Lahi'ah narrated to us, he said: Abu Hani' narrated to me, from Abu Ali, from Fudalah ibn Ubayd, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "The rider shall greet the walker, and the walker shall greet the one who is sitting, and the few shall greet the many."