Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrated: I saw the Prophet (peace and blessings be upon him) saying the takbir at every rising, lowering, standing and sitting, and he would give the salam to his right and to his left: "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)", until the whiteness of his cheek could be seen.
It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) would give the salam to his right and to his left until the whiteness of his cheek could be seen: "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)." (These are the words of the hadith of Abu Nu'aym from Sufyan.)
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 to his right until the whiteness of his cheek could be seen: "As-salamu alaykum wa rahmatullah (Peace be upon you and the mercy of Allah)", and likewise to his left.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10136