Jami` at-Tirmidhi 289

Abdullah bin Mas'ud narrated:"Allah's Messenger taught us, that when we sit for every two Rak'ah we should say: (At-Tahyyatulillah, was-salawatu wattayybaat. As-salamu alaika ayyuhannabiyyu wa rahmatullahi wa barakatuhu, as-salamu alaina wa ala ibadillahis-salihin. Ashhadu an la ilaha illallah wa ashadu anna Muhammadan abduhu wa Rasuluh.) 'All greetings, prayers, and pure words are Allah. Peace be upon you O Prophet, and Allah's mercy and His blessings. Peace be upon us and all of the righteous worshippers of Allah. I testify that nne has the right to be worshipped but Allah, and I testify that Muhammad is Allah's servant and Messenger