Sunan an-Nasa'i 1166

It was narrated from Al-Aswad and 'Alqamah, that Abdullah bin Mas'ud said:"We were with the Messenger of Allah (ﷺ) and we did not know anything, then the Messenger of Allah (ﷺ) said to us: "Every time you sit (in prayer), say: At-tahiyyatu lillahi was-salawatu wat-tayyibat, as-salamu 'alaika ayyuhan-Nabiyyu wa rahmatAllahi wa baraktuhu. As-salamu 'alaina wa 'ala 'ibad illahis-salihin, ashahdu an la illaha ill-Allah wa ashhadu anna Muhammadan 'abduhu wa rasuluhu (Allah compliments, prayers and pure words are due to Allah. Peace be upon you, O Prophet, and the mercy of Allah (SWT) and his blessings. Peace be upon us and upon the righteous slaves of Allah (SWT). I bear witness that none has the right to be worshipped except Allah and I bear witness that Muhammad is His slave and Messenger