It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with him) that Sayyiduna Umar (may Allah be pleased with him) said to him: O boy! Have you heard any hadith from the Messenger of Allah (peace and blessings be upon him) or from any of his companions regarding what a person should do if he has doubt in his prayer? While this conversation was taking place, Sayyiduna Abdur Rahman bin Awf (may Allah be pleased with him) arrived and asked: What are you discussing? Sayyiduna Umar (may Allah be pleased with him) said: I have asked this boy whether he has heard any such hadith from the Messenger of Allah (peace and blessings be upon him) or from any companion, that when a person has doubt in his prayer, what should he do? Upon hearing this, Sayyiduna Abdur Rahman (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: When any of you has doubt in his prayer and does not know whether he has prayed one rak‘ah or two, he should consider it as one rak‘ah; if he does not know whether he has prayed two rak‘ahs or three, he should consider them as two; and if he does not know whether he has prayed three rak‘ahs or four, he should consider them as three. Then, when he is about to finish the prayer, he should perform two prostrations while sitting, before the salam.

Sayyiduna Uthman bin Affan (may Allah be pleased with him) narrates that a man came to the Noble Prophet (peace and blessings be upon him) and said: O Messenger of Allah! I performed the prayer but (became forgetful), so now I do not know whether I have prayed an even number of rak‘ahs or an odd number? The Prophet (peace and blessings be upon him) said: I warn myself against this, that Satan should begin to play with you in your prayer. If any one of you performs the prayer and does not know whether he has prayed an even number of rak‘ahs or an odd number, then he should perform two prostrations; through them his prayer will be completed.

(Second chain) Marrah bin Ma’bad says: Yazid bin Abu Kashbah led us in the Asr prayer, and after completing the prayer, he turned towards us and said: I had performed the prayer with Marwan bin Hakam, and he had performed two prostrations in the same manner. Then he turned towards us and informed us that he had performed the prayer with Sayyiduna Uthman bin Affan (may Allah be pleased with him), and he had narrated from the Noble Prophet Muhammad (peace and blessings be upon him), then he narrated a hadith similar to the one above.

It is narrated from Sayyiduna Abdullah bin Mas’ud (may Allah be pleased with him), he says: The Messenger of Allah (peace and blessings be upon him) led a prayer, I do not remember whether there was an addition or a deficiency. When the Prophet (peace and blessings be upon him) finished the prayer with salam, someone said: O Messenger of Allah! Has something new been prescribed in the prayer? He (peace and blessings be upon him) said: (No, and what is it (that you are sensing)? The people said: You led such and such number of units in the prayer. So the Prophet (peace and blessings be upon him) folded his legs and performed two prostrations of forgetfulness (sahw), then when he finished with salam, he said: I am only a human being, just as you forget, I also forget. When any of you is in doubt during the prayer, he should seek (the correct number of units in the prayer), and when he finishes with salam, he should perform two prostrations.

(Second chain) There is a narration of the same kind, except that in it is: So the Prophet (peace and blessings be upon him) bent his foot, faced the Qiblah, and performed two prostrations, then turned towards us and said: If anything new had been prescribed in the prayer, I would certainly have informed you. The fact is that I am only a human being; I forget just as you forget. So if I forget, remind me. And if any one of you is uncertain in his prayer, he should seek the situation that is closest to correctness, then complete his prayer and after saying the salam, perform two prostrations.

It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: When you are in prayer and you doubt whether you have prayed three or four rak'ahs, but your predominant thought is that it is four, then recite the tashahhud and, before the salam, perform two prostrations while sitting, then recite the tashahhud again and after that, say the salam.

(Second chain) Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: When you have doubt in your prayer while you are sitting and you cannot determine whether you have prayed three rak'ahs or four, then if your predominant assumption is that you have prayed three rak'ahs, stand up and perform one rak'ah, then say salam, then perform two prostrations, then recite tashahhud, and then say salam again. And if your predominant assumption is that you have prayed four rak'ahs, then say salam, then perform two prostrations, then recite tashahhud, and then say salam again.

It is narrated from Sayyiduna Abu Hurairah, may Allah be pleased with him, that the Noble Prophet, peace and blessings be upon him, said: When any one of you is in prayer, Satan comes to him and confuses his prayer to such an extent that he does not even know how much he has prayed. So, whoever finds himself in such a situation should perform two prostrations while he is sitting.

Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) reports that the Messenger of Allah (peace and blessings be upon him) said: When any one of you prays and forgets to such an extent that he does not know how much he has prayed, then he should perform two prostrations while sitting. And when Satan comes to someone and says, "You have broken your ablution," he should say to him, "You have lied," unless he perceives a smell from his nose or hears a sound with his ear.

It is also narrated from Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: When any of you has doubt in prayer and does not know how much he has prayed, then he should base it on certainty (and perform an additional rak‘ah) until he is certain that the prayer is complete. Then, before the salam, he should perform two prostrations. If his (extra performed) prayer is odd-numbered, it will become even due to these two prostrations, and if his prayer is already even (i.e., complete), then these prostrations will humiliate Satan.