Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: Of all that I have forgotten, I have not forgotten that the Messenger of Allah (peace and blessings be upon him) would give the salam to his right: "As-salamu alaykum wa rahmatullah (Peace be upon you and the mercy of Allah)", until the whiteness of his 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 cheek could be seen.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10140