What should worshipers do if the imam forgets to recite the Tashahhud, and is their prayer valid?

If the Imam forgets to sit for the Tashahhud, there are two possibilities:

1. Forgetting to sit for the first Tashahhud: If the Imam stands up straight before the congregants realize, they should follow him and not sit for the Tashahhud. However, if they realize before he fully stands up, they should say "Subhanallah" (Glory be to Allah) to him. If he returns, then all is well. If he does not return, they should sit for the Tashahhud and then follow him. It is not permissible for a congregant to lag behind the Imam for the Tashahhud after the Imam has fully stood up.

2. Forgetting the final Tashahhud: This Tashahhud is a pillar [of prayer]. The congregants should say "Subhanallah" to the Imam. If he returns and prays the Tashahhud, the prayer of everyone is valid. However, if he does not return, they must perform the Tashahhud themselves. They have the choice to make the salutation (taslim) for themselves or wait for the Imam to make the salutation with him. If a long time passes and the Imam does not perform the Tashahhud, his prayer becomes invalid, and the prayer of the congregants who followed him in abandoning it also becomes invalid.