If he stood up or was half way, he has to offer sujood assahu. But if he didn’t stand up but was almost to do so, he doesn’t need to prostrate for forgetfulness.

Suppose an Imam finishes the second prostration of the fourth rakah in Thuhr prayer and he mistakenly starts going to the standing position; but before he can do so, the congregation reminds him (by saying ‘SubhanAllaah’); he then sits down for the tashahudd. Are the two prostration of forgetfulness a must in this case?