What is the correct form of greeting?

There are authentic hadiths from the Prophet, peace be upon him, regarding the tashahhud. The strongest of these, by agreement of the hadith scholars, is the hadith of Ibn Mas'ud, who said: "We used to pray behind the Prophet, peace be upon him, and say: 'Peace be upon Allah.' The Prophet, peace be upon him, then said: 'Indeed, Allah is As-Salam (The Peace). But 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 good things are due to 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).' And people have agreed upon the tashahhud of Ibn Mas'ud."

Among the other authentic forms of tashahhud is what Muslim narrated from Ibn Abbas, and its wording is: "At-tahiyyatul-mubarakatus-salawatut-tayyibatu lillah. As-salamu 'alayka ayyuhan-nabiyyu wa rahmatullahi ta'ala wa barakatuh. As-salamu 'alayna wa 'ala 'ibadillahis-salihin. Ashhadu an la ilaha illallah, wa ashhadu anna Muhammadan rasulullah (All greetings, blessings, prayers, and good things are due to 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 the Messenger of Allah)." Imam Shafi'i chose this wording.