It is narrated from Sayyiduna Zayd bin Arqam (may Allah be pleased with him); he said: When the Prophet (peace and blessings be upon him) gave us the salam at the end of the prayer, we would say: "Wa alaykum as-salamu wa rahmatullahi wa barakatuh (and upon you be peace, and the mercy of Allah and His blessings)."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4996