Musannaf Ibn Abi Shayba 3:611

Abu Usama narrated to us, from Sa'id, who said: Qatada narrated to me, from Yunus ibn Jubayr, from Hattan ibn Abdullah, from Abu Musa, from the Prophet (may Allah bless him and grant him peace), who said: "When it is the sitting, let one of you say: 'The greetings (at-tahiyyat) are for Allah, the pure things (at-tayyibat), the prayers (as-salawat) are for Allah. 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.'"