Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) would give the salam to his right and to his left until the whiteness of his face could be seen, or became visible. And of all that I have forgotten, I have not forgotten this: "As-salamu alaykum wa rahmatullah, as-salamu alaykum wa rahmatullah (Peace be upon you and the mercy of Allah, peace be upon you and the mercy of Allah)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10142