Question 268: Why do Usooli Shia Muslim consider Tashood cannot be changed with 3rd Sahada of Imam Ali AS. If azaan is added with Ali Un Wali Ullah.
Answer 268: As the prayer is Tawqifi act we must not add anything other than what shariat allowed us to do. Since, the third testimony is not a part of prayer we are not allowed to add it in tashahud.
There are different narrations regarding zikr (recital) of Tashahhud. However, all jurisprudents consider what they have mentioned in their Manuals of Islamic Laws (Resalah) as sufficient. When it comes to reciting more azkar (plural of zikr) in Tashahhud, if one recites them without the intention of wurud (i.e. doing an act which has no religious origin or validation), there is no problem in it but if he recites with the intention of wurud, some scholars have allowed reciting an additional zikr keeping in view that some of the narrations have been accepted and others have not. However, most of the jurisprudents have sufficed to the zikr that has been mentioned in the Manuals of Islamic Laws.
Makarem Shirazi (ha): The precaution is that Tashahhud has to be recited as usual.
Saafi Gulpaigani (ha): Tashahhud has to be recited in the same way that has been prescribed in the book of Islamic Laws.
Noori Hamedani (ha): There is no objection in reciting the azkar without intention of wurud.
Sayyid Sistani (ha): The third testimony must not be recited in Tashahhud, as an obligatory precaution. It is recommended to recite the following zikr the second testimony in Tashahhud: “Arsalahu bil haqqi bashiran wa naziran bayna yada yis sa’ah, wa ashhadu anna rabbi ne’mar Rab wa anna Muhammadan ne’mar Rasul and then recite the salawat.”
For further information in this regards, please refer to the following answer:
Index: “Ashhadu anna aliyyan waliyyullah” in Adhan and Iqamah, answer 276.
 . Imam Khomeini, Istiftaat, Vol. 1, Pg. 167.
 Grand Ayatollah Sistani (ha).