What should the worshipper do if he enters the mosque and the Imam is in the Tashahhud, and he does not know which Tashahhud it is, should he recite the first or the second Tashahhud?

What is prescribed for the latecomer, when he joins the Imam, is to do as the Imam does. So, if he catches the Imam in the tashahhud (testification), he should sit for the tashahhud and recite it. It is recommended for him to concur with the Imam in reciting the tashahhud and the glorifications (tasbihāt). Ibn Baz holds the view that it is recommended for the latecomer to recite the tashahhud, based on the generality of the Prophet's ﷺ saying: "The Imam is only appointed to be followed, so do not differ from him."

The Hanbalis are of the opinion that the worshipper should not add more than the first tashahhud in this situation; rather, he should repeat it if the Imam prolongs his sitting, because sending blessings upon the Prophet ﷺ is not prescribed except before the salām (final salutation) according to them.

The most preponderant view is that sending blessings upon the Prophet ﷺ is prescribed even if not in the last tashahhud, and this is the madhhab of the Shafi'is. Therefore, it is preferable for the latecomer, if he recites the tashahhud with his Imam, to send blessings upon the Prophet ﷺ and to supplicate thereafter with whatever he wishes if the Imam prolongs his sitting. He should remain in his state until the Imam stands for the third unit or performs the salām.