Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: Whatever I may have forgotten, I have not forgotten how the Messenger of Allah (peace and blessings be upon him) gave the salam in the prayer 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)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10149