Musannaf Ibn Abi Shayba 3:608

Hushaym narrated to us, he said: Husayn ibn Abd al-Rahman, and Mughirah, and al-A’mash informed us, from Abu Wa’il, from Abdullah, who said: When we used to sit behind the Messenger of Allah (peace be upon him) in prayer, we would say: "Peace be upon Allah, peace be upon Gabriel, peace be upon Michael, peace be upon so-and-so, peace be upon so-and-so." He said: The Prophet (peace be upon him) turned to us and said: "Indeed, Allah is As-Salam (the Peace). So say: 'At-tahiyyatu lillahi was-salawatu wat-tayyibat. As-salamu ‘alayka ayyuhan-nabiyyu wa rahmatullahi wa barakatuh. As-salamu ‘alayna wa ‘ala ‘ibadillahis-salihin. Ashhadu an la ilaha illallah wa ashhadu anna Muhammadan ‘abduhu wa rasuluh' (All compliments, prayers, and pure words 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)." "For if you do that, you have indeed sent peace upon every righteous servant in the heavens and the earth."