Sunan An-Nasa'i

Abdullah bin Mas'ud narrated that:The Messenger of Allah (ﷺ) used to say salam to his right:As-salamu 'alaykum wa rahmatullah (peace be upon you and the mercy of Allah) , until the whiteness of his right cheek could be seen, and to his left: As-salamu 'alaykum wa rahmatullah (peace be upon you and the mercy of Allah) until the whiteness of his left cheek could be seen