Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
The correct view is to adhere to the standard formula of tashahhud which the Prophet related to his Companions including Ibn Masud as will be explained below.
Answering your question, Dr. Mohammad S. Alrahawan, Associate Professor at the department of Islamic Studies in English, Al-Azhar University, Egypt, states:
Formula of Tashahuud
“(Tashahhud 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 Masud 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.
Yahya related from Malik from Yahya ibn Sa`id al-Ansari that al-Qasim ibn Muhammad ibn Muhammad told him that Aishah, 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. May peace be upon us and on the slaves of Allah who are salihun. Peace be upon you. ” (Al-Muwatta)
Therefore, the correct view is to adhere to the standard formula which the Prophet related to his Companions including Ibn Masud.
Allah Almighty knows best.