The scholars of the Standing Committee were asked:
It
was narrated that Ibn Mas’ood (radiAllahu 'anhu) said: The Messenger of
Allah (salAllahu 'alayhi wa salam) taught me the tashahhud with my hand
between his hands, just as he taught me a soorah from the Qur’an:
“Al-tahiyyaatu Lillaahi wa’l-salawaatu wa’l-tayyibaat al-salaamu
‘alayka ayyuha’l-nabiyyu… (All compliments, prayers and good words
are for Allah, peace be upon you, O Prophet…).” (That was) when he
was still among us. When he passed away, we started to say,
“al-salaam ‘ala al-nabi (peace be upon the Prophet (salAllahu 'alayhi wa salam)).”
Many people say the latter version and teach others to do likewise.