Ibn Mas‘ūd (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) taught me the Tashahhud, with my hand between his two hands, as he would teach me a Surah from the Qur’an: "At-tahiyyātu lillāhi was-salawātu wat-tayyibātu as-Salāmu ‘alayka ayyuha an-nabiyyu wa rahmatullāhi wa barakātuh, as-salāmu ‘alayna wa ‘ala ‘ibādillāhi as-sālihīn, ash-hadu alla ilāha illallāh wa ash-hadu anna Muhammadan ‘abduhu wa rasūluh (Greetings, prayers, and good things are due to Allah. May the peace, mercy, and blessings of Allah be upon you, O Prophet. Peace be upon us and upon the righteous slaves of Allah. I bear witness that there is no god but Allah, and I bear witness that Muhammad is His slave and Messenger)." Another wording reads: "Verily, Allah is the Giver of Peace, so when any of you sits in prayer, let him say: 'At-tahiyyātu lillāhi was-salawātu wat-tayyibātu as-salāmu ‘alayka ayyuha an-nabiyyu wa rahmatullāhi wa barakātuh, as-salāmu ‘alayna wa ‘ala ‘ibādillāhi as-sālihīn (Greetings, prayers, and good things are due to Allah. May the peace, mercy, and blessings of Allah be upon you, O Prophet. Peace be upon us and upon the righteous slaves of Allah).' If you say this, it will benefit every righteous slave of Allah in heaven and on earth. (Then, say:) 'Ash-hadu alla ilāha illallāh wa ash-hadu anna Muhammadan ‘abduhu wa rasūluh (I bear witness that there is no god but Allah, and I bear witness that Muhammad is His slave and Messenger).' Then, let him choose whatever supplication he would like."
Authentic hadith - Narrated by Bukhari & Muslim