Al-Nakhai, al-Awza'i, and those who forgot a prostration, then remembered it, perform it within the prayer whenever they remember it. Al-Awza'i said: He returns to the position he was in during the prayer at the time he remembered it, and continues from there. The People of Opinion (Ashab al-Ra'y) said, regarding someone who forgot four prostrations from four rak'ahs then remembered them during the tashahhud: He performs the four prostrations immediately, and his prayer is complete. Our position is that the one crowded in the Friday prayer, if the crowding ceases while the Imam is bowing in the second rak'ah, he follows him and prostrates with him, and the prostration is counted for the second rak'ah, not the first. The same applies here.

Section: If he proceeds in a position where returning is required, or returns in a position where proceeding is required, while knowing the prohibition of that, his prayer is invalidated; because he intentionally omitted a duty in the prayer. If he does that believing it to be permissible, it is not invalidated; because he omitted it without intent, similar to one who proceeds before remembering the omitted part. However, if he proceeds in a position where he is required to return, the rak'ah from which he omitted the pillar is invalidated, just as if he had not remembered it until after starting the [recitation. And if he returns in a position where he should have proceeded, he does not count what he performs in the rak'ah from which he omitted it, because it was invalidated by his beginning the] (52) recitation of something else, so it cannot return to validity under any circumstances. The third scenario: He stood up from the final tashahhud to [an additional rak'ah] (53), so he returns to it whenever he remembers; because he stood up for an addition that does not count for him. Thus, returning is required of him, just as if he had remembered before the prostration. The details of this scenario will come in the case where one prays five rak'ahs. In these three scenarios, he is required to perform prostration before the taslim.

Section: His saying: "Or he sat in a position of standing." This is envisioned by him sitting immediately after the first or third [rak'ah], thinking it is the position for the tashahhud or the intermediate sitting, so he stands whenever he remembers. And if he does not remember until he has stood up, he completes his prayer and performs the prostration of forgetfulness; because he added to the prayer something of its own nature which, if done intentionally, would invalidate it. Therefore, prostration is required of him if it was out of forgetfulness, like adding a rak'ah.