Musnad Ahmad 26:327

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Abu Ishaq, from Abu al-Ahwas, from Ibn Mas'ud, who said: The Messenger of Allah (peace and blessings of Allah be upon him) taught the beginnings of goodness and its comprehensive expressions—or the comprehensive expressions of goodness and its beginnings—and we did not know what to say in our prayer until he taught us. He said: "Say: 'The greetings (al-tahiyyat) are for Allah, and the prayers (al-salawat) and the good things (al-tayyibat). 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 is His servant and His messenger.'"