Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: I saw the Messenger of Allah (peace and blessings be upon him) giving the salam: "As-salamu alaykum wa rahmatullah (Peace be upon you and the mercy of Allah)", until the whiteness of his cheek could be seen, and likewise on the other side.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10139