What is the ruling if the imam intentionally omits sitting for the first tashahhud in a four-rak'ah prayer?

The questioner : Sitting in a four-rak'ah prayer: If the imam intentionally did not sit in a four-rak'ah prayer, what is the ruling on that?

The Shaykh : You mean he did not sit for the first tashahhud.

The questioner : Yes, that is right.

The Shaykh : This is a follow-up to the question about leaving an obligatory act. If he did it intentionally, he is sinful. If he did it out of forgetfulness, he must perform the prostration for forgetfulness. If he did it intentionally, he is sinful, sinful.
The student : Does that invalidate the prayer, O Shaykh?

The Shaykh : He is sinful.
The student : There are people who say that this is a rest.

The Shaykh : Okay, what do we do with them if they say that? What does it mean?
The student : There are people who say that sitting for the first tashahhud in a four-rak'ah prayer is a rest, so there is no blame on him if he does not sit.

The Shaykh : I think the one who says this, at least when he said it, did not have in mind the saying of the Messenger of Allah (peace and blessings be upon him): So he is shaping the prayer according to his own will and philosophizing it with his own philosophy, saying it is a rest. We will say: suppose it is a rest, is this rest obligatory, a recommended practice, or are you given a choice in it?
The student : It is obligatory, O Shaykh.

The Shaykh : He must give an answer, but the one who says such a thing has no answer because he has no knowledge. If he had knowledge, he would not have uttered what he uttered.
The student : May Allah reward you with good.

The Shaykh: Yes.