Wa `alaykum as-salam wa rahamatullah wa barakatuh

According to Sahih Al-Bukhari, Ibn Mas`ud narrated: Allah’s Messenger (peace be upon him) taught me the tashahhud 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 (peace be upon him) , but when he had died, we used to say, “Peace be on the Prophet.” (Al-Bukhari)

Scholars maintained that this is the view of `Abdullah ibn Mas`ud which is based on his own personal reasoning. It has not been accredited by other Companions of the Prophet. For example, `Umar recited the standard formula of tashahhud on the pulpit, where the Prophet’s companions received it without objection. It has been related in Malik’s Muwatt. Yahya related to from Malik from Yahya ibn Sa`id al-Ansari that al-Qasim ibn Muhammad ibn Muhammad told him that `A’ishah, the wife of the prophet, may Allah bless him and grant him peace, used to say in the tashahhud, “Greetings, good words, prayers, pure actions belong to Allah. I testify that there is no god except Allah, alone without partner, and I testify that Muhammad is the slave of Allah and His Messenger. Peace be upon you, Prophet, and the mercy of Allah and His blessings. Peace be upon us and on the slaves of Allah who are salihun. Peace be upon you. ” (Al-Muwaṭṭa’)

Therefore, the correct view is to adhere to the standard formula which the Prophet related to his Companions including Ibn Mas`ud.