Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: We used to say in the prayer: Peace be upon Allah. The Prophet (peace and blessings be upon him) heard this and said: "Do not say: Peace be upon Allah, for Allah Himself is al-Salam. Rather, say: All verbal, physical and financial acts of worship are for Allah alone. Peace be upon you, O Prophet, 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 god but Allah, and I bear witness that Muhammad is the servant of Allah and His Messenger."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9854