If the imam forgets the first tashahhud in a four-rak'ah prayer and the ma'mum remind him, what should he do?

Question:
If the imam forgets and does not sit for the first tashahhud in a four-rak'ah prayer, and the ma'mum remind him, is it permissible for him to sit, or should he complete his prayer? In that case, how is the matter handled? May Allah reward you with good. If he says, "I am going to the third rak'ah" in 'Asr, or Dhuhr, or Maghrib, or 'Isha, meaning what?

Answer:
In a two-rak'ah prayer, if he stands for the third rak'ah without sitting for the first tashahhud, and then he remembers or is reminded before he is fully established in standing, it is obligatory for him to return and perform the first tashahhud, because it is one of the obligatory acts of the prayer. However, if he is fully established in standing, it is disliked for him to return, whether he remembers or is reminded; it is disliked for him to return because he has begun the second rak'ah. As for if he has begun the recitation, in reciting al-Fatihah, it is forbidden for him to return, because he has begun the second rak'ah and its recitation. It is not permissible for him to return from a pillar in which he has begun in order to perform an obligatory act he neglected. But he must continue and complete his prayer. Then, when he intends to say the taslim, he performs two prostrations of forgetfulness to compensate for the deficiency that occurred in his prayer. This is the sunnah (recommended practice) of the Messenger of Allah (peace and blessings be upon him). May Allah reward you with good.