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 Jibril. The Prophet (peace and blessings be upon him) heard us saying this and said: "Allah Himself is al-Salam. So when any of you sits in the prayer, let him 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."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9849