Mustadrak al-Hakim — Hadith 994

Sayyiduna Umar bin al-Khattab (may Allah be pleased with him), while delivering the sermon from the pulpit of the Messenger of Allah (peace and blessings be upon him), used to teach the people the tashahhud of the prayer, saying: When one of you recites the tashahhud, let him say: 'Bismillahi khayril-asma', at-tahiyyatuz-zakiyatus-salawatut-tayyibatu lillah, as-salamu 'alayka ayyuhan-Nabiyyu wa rahmatullahi wa barakatuh, as-salamu 'alayna wa 'ala 'ibadillahis-salihin, ashhadu an la ilaha illallahu wahdahu la sharika lahu wa anna Muhammadan 'abduhu wa rasuluh' (In the name of Allah, the best of names. All verbal, pure, bodily and financial worship is for Allah alone. 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, alone, with no partner, and that Muhammad (peace and blessings be upon him) is His servant and Messenger). Sayyiduna Umar also said: After sending salam upon the Messenger of Allah (peace and blessings be upon him), begin with yourselves, and then send peace upon the righteous servants of Allah.
This hadith is sahih on the condition of Muslim, and I have mentioned it because it has supporting narrations (shawahid).