Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: In the beginning, when we sat between the two rak'ahs, we would say tasbih and takbir and remember Allah because we did not know (what to say). Indeed, the Messenger of Allah (peace and blessings be upon him) knew the best of words. He turned towards us and said: "When you sit between the two rak'ahs, say: 'All verbal, practical 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 (peace and blessings be upon him) is the servant of Allah and His Messenger.'" Sayyiduna Abdullah (may Allah be pleased with him) says: When a person says 'As-salamu 'alayna wa 'ala 'ibadillahis-salihin' (Peace be upon us and upon the righteous servants of Allah), that salam reaches every righteous servant, or he said: every Prophet sent as a Messenger. Then let him begin with the praise of Allah and extol and praise Allah as He deserves, then send blessings upon the Prophet (peace and blessings be upon him), and then after that ask Allah for whatever he wishes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2855