Introduction to Fiqh-us-Sunnah
It is confirmed that the Prophet sallallahu alehi wasallam sometimes forgot something in the salah. It is also true that he said: "I am a human being and forget like you forget. If I forget, remind me." There are specific points concerning such prostrations and they are presented below.
Volume 2, Page 46a: How to perform these prostrations of forgetfulnessThe "prostrations of forgetfulness" (sujjud us-sahu) are two prostrations which a person makes before the taslim. All of this has been confirmed from the Prophet sallallahu alehi wasallam. In the sahih, it is recorded from Abu Sa'id al-Khudri that the Prophet said: "If one of you has some doubts during his salah and he does not recall (the number of rak'at) he has prayed, three or four, then he can put an end to his doubt by performing salah according to what he was certain of [the lesser amount] and then making two sujjud before the taslim." In the story of Zhul-Yadain, in the two Sahihs, we are told the Prophet sallallahu alehi wassalam made the prostrations after the taslim.
Ash-Shaukani says: "The best that is stated on this subject is that one must follow what the Prophet said or did, respecting the sujjud before or after the taslim. If one does something that necessitates sujjud before the taslim, one should make them before the taslim, and if one does something requiring sujjud after the taslim, then one should make them after the taslim. As for those acts of forgetfulness that are not related to any specific time either before or after the taslim, one may choose to make the prostrations before or after the taslim in cases of addition or reduction in the salah. This is based on what Muslim recorded in his Sahih from Ibn Mas'ud that the Prophet sallallahu alehi wasallam said: 'If one adds or decreases something from his salah, he should make two sujjud."'
Volume 2, Page 47: When to perform these prostrations of forgetfulnessThe "sujjud us-sahu" are to be performed in the following circumstances:
-1- If a person makes the taslim before he actually completes the prayers. Ibn Sireen relates from Abu Hurairah who said: "The Prophet sallallahu alehi wasallam prayed either zuhr or 'asr salah with us and he prayed only two rak'at and made the taslim. He got up and leaned against a piece of wood in the mosque as if he was angry. He put his right hand on his left and interlocked his fingers. Then, he placed his cheek on the back of his left hand. And some people left the mosque in a hurry. And they said: 'The prayer has been shortened?' Among the people were Abu Bakr and 'Umar, and they were shy to speak to him. One of the people, who was called Zhul-Yadain, said: 'O Messenger of Allah have you forgotten or has the prayer been shortened?' He answered: 'I have not forgotten and it has not been shortened.' Then he asked: 'Is it as Zhul-Yadain has said?' The people answered in the affirmative...At that, he led the people in what he had ommitted and made the taslim. After which he made the takbir and prostrated the way he usually prostrated or perhaps even longer. Next, he raised his head and made the takbir. Then, he made the takbir [again] and prostrated, like one of his customary sujjud or perhaps even longer, and finally, he raised his head." This is related by al-Bukhari and Muslim.
'Ata' relates that Ibn az-Zubair prayed maghrib and made the taslim after two rak'at and then he stood up and wanted to kiss the black stone, when the people tried to correct him he said: "What is the matter with you?" Then he prayed what he had left out and performed two sujjud. When this was mentioned to Ibn 'Abbas, he said that it was not far from the sunnah of the Prophet sallallahu alehi wasallam. This is related by Ahmad, al-Bazzar, and at-Tabarani.
-2- In the case of an addition to the prayer. Ibn Mas'ud narrates that the Prophet prayed five rak'at and the people asked him: "Has there been an addition to the prayer?" He asked: "Why do you say that?" They replied: "You prayed five rak'at" Then he made two sujJud after he had made the taslim. This is related by the group. This hadith proves that the prayer of one who prays five rak'at out of forgetfulness, without sitting during the fourth rak'ah, is acceptable.
-3- In the case of forgetting the first tashahud or one of the other sunnah acts of the prayer. Ibn Buhainah narrates that the Prophet stood after two rak'at. The people tried to correct him but he continued. When he finished his salah, he made two sujjud and made the taslim. This is related by the group.' This hadith shows that one who forgets the first sitting but is reminded of it and he recalls it before he completely stands should return and sit, but if he is already completely standing, he should not sit down. This is supported by what Ahmad, Abu Dawud, and Ibn Majah recorded from al-Mughirah ibn Shu'bah, that the Prophet sallallahu alehi wasallam said: "If one of you stands after two rak'at and he has not completely stood, then he should sit. If he is already completely standing, he should not sit and he should make two sujjud of forgetfulness."
-4- In the case of doubt over whether or not one performed some act of the prayer. 'Abdurrahman ibn 'Auf reported that he heard the Prophet say: "If one of you has some doubt during his salah and he does not know if he prayed one rak'ah or two, he should take it to have been just one. If he does not know if he prayed two rak'at or three, he should take it to have been just two. If he does not know if he prayed three rak'at or four, he should take it to have been just three. [In all such cases] at the end of his prayer, while sitting, he should make two sujjud before the taslim." This is related by Ahmad, Ibn Majah, and at-Tirmizhi. The latter grades it sahih.
In one narration, it is stated: "Whoever prays and has some doubt that he was short of the complete prayer, he should continue praying until he suspects that he has added something to the prayer [with respect to the number of rak'at that he has prayed]." Abu Sa'id al-Khudri narrated that the Prophet sallallahu alehi wasallam said: "If one of you has some doubts during his prayer and does not know if he prayed three or four [rak'at], then he should remove his doubt by praying according to the amount that he is certain he had performed and then make two sujjud before the taslim. If he had prayed five rak'at, the two sujjud would make it even. If he had prayed a complete four rak'at [when he had finished], they would be in defiance of the Satan." This is related by Ahmad and Muslim. These two ahadith prove what the majority of the scholars have said, namely, if one has some doubt concerning the number of rak'at one has prayed, one should act according to the amount that one is certain to have prayed (the lesser amount) and then make two sujjud before the taslim.