What is the ruling on someone who becomes confused while reciting the Tashahhud regarding the different versions of its wording, and if he remembers the mistake after finishing the prayer or before the Taslim, what should he do?

A Muslim should preserve the wordings of the prescribed ذكر (remembrance) found in prayer and elsewhere. The Prophet (peace and blessings be upon him) used to teach the Companions the Tashahhud just as he would teach them a Surah from the Qur'an, which indicates the great care taken with the wordings of the Tashahhud.

If the worshipper says in the Tashahhud: (And I bear witness that Muhammad is the Messenger of Allah) instead of (And I bear witness that Muhammad is His slave and His Messenger), their prayer is valid, because this does not deviate from the prophetic wording. However, if they mix up the Tashahhud in a way that alters the meaning, then if this occurs before the Taslim (salutation) of the prayer, they should repeat the Tashahhud correctly. If it occurs after the Taslim of the prayer and the time is still short, they should return to the Tashahhud and repeat it, then perform the Taslim and prostrate for forgetfulness (Sujud al-Sahw). If it occurs after a long interval, they must repeat the entire prayer. If the worshipper makes a mistake or forgets in the final Tashahhud, they should repeat from where they erred, then complete what they erred in and what follows it, because order is essential.