The companions of Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) heard him say: We did not know what to say in the prayer until the Messenger of Allah (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."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9878