Muslim

974. Commentary:
➊ “He used to teach the tashahhud with such care as he would teach the Qur’an.” This indicates that it is obligatory. The translation is similar to the words mentioned above, i.e., all blessed greatness and pure praises are exclusively for Allah.
➋ It is explicitly stated by Ibn Abbas (radi Allahu anhuma) that the Prophet (sallallahu alayhi wa sallam) himself used to recite the entire tashahhud with these very words which he (sallallahu alayhi wa sallam) taught to the noble Companions (radi Allahu anhum ajma‘in).

Benefits and Issues:
➊
The meaning of teaching this supplication (dua) like the Qur’an is that it was taught with great care and attention.
From this, it is understood that this supplication must certainly be recited in prayer.

➋
Just as it is not permissible to add to or subtract from the words of the Qur’an, but some words have been revealed in multiple forms,
and it is correct to recite them in any of those forms,
similarly, those supplications which have been narrated in multiple ways
may be recited in any of the narrated forms.

➌ (Ayyuha an-Nabi)
The purpose of “O Prophet!” is not to make the Messenger of Allah (sallallahu alayhi wa sallam) hear,
rather, these words are recited in the same manner as the words of the Noble Qur’an are recited.
For example: (Ya Nuh, Ya Ibrahim, Ya ayyuhal-muzzammil, Ya ayyuhalladhina amanu, Ya ayyuhan-nas, Ya Bani Adam, Ya Fir‘awn, Ya Haman),
etc.
While reciting them, the reciter does not intend to address them nor consider them present or in attendance.

1279. Commentary: In prescribed and Sunnah supplications and remembrances (adhkar and awrad), one should avoid, as much as possible, any addition, omission, or alteration—even to the extent that one should not say the word "Rasul" in place of the word "Nabi." This is the very meaning of teaching as in the Qur'an. Similarly, the adhan and the prescribed supplications (ad‘iyah masnunah) should be recited exactly as they are; otherwise, one may be accused of alteration (tahrif). However, general supplications (du‘a) may be made according to one's preference, although the supplications transmitted in the Qur'an and Hadith are, in any case, comprehensive, blessed, and superior.

Chapter 43: How should the tashahhud be recited?