Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: We used to pray behind the Messenger of Allah (peace and blessings be upon him), and we would say: Peace be upon Allah. So he (peace and blessings be upon him) said: "Allah is Himself al-Salam (Peace); rather, say: All verbal, physical and monetary acts of worship are for Allah alone. O Prophet, peace be upon you, and the mercy of Allah and His blessings. Peace be upon us and upon the righteous servants of Allah. I bear witness that there is no deity except Allah, and I bear witness that Muhammad is the servant of Allah and His Messenger."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9866