Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: When we sat in the prayer, we used to say: Peace be upon Allah, peace be upon us from our Lord, peace be upon Jibril and Mika'il, peace be upon so-and-so. Then the Messenger of Allah (peace and blessings be upon him) said: "Allah Himself is al-Salam. When you sit in the prayer, 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. When this is said, it reaches every righteous servant, whether in the heaven or on the earth. I bear witness that there is no god but Allah, and I bear witness that Muhammad (peace and blessings be upon him) is His servant and His Messenger. Then (the one praying) may choose whatever words (of supplication) he wishes."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9850