Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: We did not know what to say in every rak'ah; we would glorify, magnify and praise our Lord. And Muhammad (peace and blessings be upon him) knew the opening and closing matters of goodness - or he said: its comprehensive matters - and he (peace and blessings be upon him) said to us: "When you sit after every two rak'ahs, 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. Then let him choose from among the supplications the one he likes most, and make that supplication."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9876