Masruq (may Allah have mercy on him) narrates from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that he said: Whatever things I may have forgotten, I have not forgotten how the Messenger of Allah (peace and blessings be upon him) gave the salam in prayer to his right and to his left. Rather, I remember well that he (peace and blessings be upon him) would give the salam in prayer to his right and to his left, and he (peace and blessings be upon him) would say: 'As-salamu alaykum wa rahmatullah, as-salamu alaykum wa rahmatullah' (Peace and the mercy of Allah be upon you, peace and the mercy of Allah be upon you). Then he (may Allah be pleased with him) said: It is as though I am (still) looking at the whiteness of the blessed face of the Prophet (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2977