What is the ruling on an imam who left a prostration in the prayer?

The questioner : An imam prayed and did not perform the second prostration, and someone behind him alerted him, but he did not respond and continued his prayer?

The Shaykh: Is the question clear?

The questioner: Yes.

The Shaykh : Okay, and what did he do after he finished the prayer?

The questioner : He performed the prostration for forgetfulness.

The Shaykh : No, that is not sufficient. It is obligatory for him to repeat the unit of prayer or he must perform the prostration.

The questioner : He must perform the prostration, yes.

The Shaykh : Yes, after that he performs the two prostrations for forgetfulness.

The questioner : Is the unit of prayer considered invalid?

The Shaykh : It is considered an invalid unit of prayer.

The questioner : One person spoke and said that one of the worshippers should volunteer and say, "O Shaykh, you forgot a unit of prayer," or one person should hit him from behind?

The Shaykh : Hit by whom?

The questioner : The imam, meaning he alerts him that you forgot a prostration?

The Shaykh : During the prayer or after the prayer?

The questioner: During the prayer.

The Shaykh-Laughter- How does he alert him?

The questioner : Meaning, one person behind him pokes him in the back.

The Shaykh : But does he understand from the poke that he made a mistake?

The questioner : Meaning, one person volunteers to invalidate his prayer for the sake of the worshippers?

The Shaykh : It is not permissible; it is forbidden.