Silsilat al-Ahadith al-Sahihah

Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: We did not know what to say (in prayer when sitting after two) rak'ahs, except that we used to glorify Allah, proclaim His greatness, and praise Him. Indeed, Muhammad, the Messenger of Allah (peace and blessings be upon him), was taught the beginning and end of all goodness. So (one day) the Prophet (peace and blessings be upon him) said: "When you sit after two rak'ahs, recite: All verbal, physical, and financial acts of worship are for Allah. 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 worthy of worship except Allah, and Muhammad (peace and blessings be upon him) is His servant and Messenger. (After reciting this tashahhud) he should choose whatever supplications he likes."