Does prostration of forgetfulness (before or after the salām) or repeating the prayer become necessary for someone who completed the entire Tashahhud after the second Rakʿah of a four-Rakʿah prayer?

The prayer of one who completes the Tashahhud in the middle sitting is valid, and he is not required to perform the prostration of forgetfulness (Sujud al-Sahw). The majority of jurists do not obligate it in this case; rather, it is recommended but not obligatory.

Adding a prescribed remembrance (dhikr) in the prayer in an inappropriate place does not invalidate the prayer. An example of this is sending blessings upon the Prophet (peace be upon him) in the first Tashahhud.

It is important to note the significance of reciting: "I bear witness that there is no god but Allah, and I bear witness that Muhammad is His servant and Messenger" as an essential part of the Tashahhud.

It is preferable to adhere to the wording transmitted from the Prophet (peace be upon him) in the remembrances and supplications, especially in the Tashahhud, and to avoid additions that have not been narrated, such as the words "al-Kareem" (the Generous) or "Sayyiduna" (our Master), even though this does not affect the validity of the prayer.