From Sahih Bukhari (Hadith 6265):
Narrated Ibn Mas`ud:Allah's Messenger (ﷺ) taught me the Tashah-hud as he taught me a Sura from the Qur'an, while my hand was between his hands. (Tashah-hud was) all the best compliments and the prayers and the good things are for Allah. Peace and Allah's Mercy and Blessings be on you, O Prophet! Peace be on us and on the pious slaves of Allah, I testify that none has the right to be worshipped but Allah, and I also testify that Muhammad is Allah's slave and His Apostle. (We used to recite this in the prayer) during the lifetime of the Prophet (ﷺ) , but when he had died, we used to say, "Peace be on the Prophet
Reference: Bukhari 6265